The UFO Connection
1/2Marvel Preview 13 (1978/wi)
2/2Marvel Preview 13 (1978/wi)
Ultimate Enemy
1/4Ultimate Enemy 1 (2010/03)
2/4Ultimate Enemy 2 (2010/04)
3/4Ultimate Enemy 3 (2010/06)
4/4Ultimate Enemy 4 (2010/07)
Ultimate Knights
Ultimate Starts
Ultimates vs. Ultimate X-Men
1/4Ultimate War 1 (2003/02)
2/4Ultimate War 2 (2003/02)
3/4Ultimate War 3 (2003/03)
4/4Ultimate War 4 (2003/04)
Ultimatum (A)
Ultimatum (B)
1/3Ultimate X-Men 98 (2009/01)
2/3Ultimate X-Men 99 (2009/02)
3/3Ultimate X-Men 100 (2009/04)
[Ultron Initiative]

Name of story arc taken from TPB. Personally, I would have voted for "This Evil Uncovered".

1/6Mighty Avengers 1 (2007/05)
2/6Mighty Avengers 2 (2007/06)
3/6Mighty Avengers 3 (2007/07)
4/6Mighty Avengers 4 (2007/08)
5/6Mighty Avengers 5 (2007/11)
6/6Mighty Avengers 6 (2008/02)
Ultron Unlimited
PreludeAvengers (III) 0 (1999/07)
1/4Avengers (III) 19 (1999/08)
2/4Avengers (III) 20 (1999/09)
3/4Avengers (III) 21 (1999/10)
4/4Avengers (III) 22 (1999/11)
Under Fire
Under My Skin
The Underbase Saga!
1/4Transformer (US) 47 (1988/12)
...Transformers (UK) 194 (1988/12)
...Transformers (UK) 195 (1988/12)
2/4Transformer (US) 48 (1989/01)
...Transformers (UK) 196 (1988/12)
...Transformers (UK) 197 (1988/12)
3/4Transformer (US) 49 (1989/02)
...Transformers (UK) 206 (1989/02)
...Transformers (UK) 207 (1989/03)
4/4Transformer (US) 50 (1989/03)
...Transformers (UK) 208 (1989/03)
...Transformers (UK) 209 (1989/03)
...Transformers (UK) 210 (1989/03)
Underboss
1/6Daredevil (II) 26 (2001/12)
2/6Daredevil (II) 27 (2002/01)
3/6Daredevil (II) 28 (2002/02)
4/6Daredevil (II) 29 (2002/03)
5/6Daredevil (II) 30 (2002/04)
6/6Daredevil (II) 31 (2002/05)
The Underground (A)
PreludeWeapon X (II) 6 (2003/04)
1/7Weapon X (II) 7 (2003/05)
2/7Weapon X (II) 8 (2003/06)
3/7Weapon X (II) 9 (2003/07)
4/7Weapon X (II) 10 (2003/08)
5/7Weapon X (II) 11 (2003/09)
6/7Weapon X (II) 12 (2003/10)
7/7Weapon X (II) 13 (2003/11)
The Underneath (B)
1/2Ultimate X-Men 82 (2007/07)
2/2Ultimate X-Men 83 (2007/08)
Underneath the Skin
Unfair
Unfinished Business (B)
1/3New Mutants (III) 25 (2011/07)
2/3New Mutants (III) 26 (2011/08)
3/3New Mutants (III) 27 (2011/09)
Unfinnished Business
1/2Cable/Deadpool 36 (2007/03)
2/2Cable/Deadpool 37 (2007/04)
Unintended Consequences
Unions
Universal Laws
1/2She-Hulk (I) 7 (2004/11)
2/2She-Hulk (I) 8 (2004/12)
The Universe According to Thanos
1/2Silver Surfer (III) 55 (1991/09)
2/2Silver Surfer (III) 56 (1991/10)
Unknown Jungle
Unlucky Bucks
1/4Marvel Tales Annual 201 (1987/07)
2/4Marvel Tales Annual 202 (1987/08)
3/4Marvel Tales Annual 203 (1987/09)
4/4Marvel Tales Annual 204 (1987/10)
Unnatural
1/5Mystique 14 (2004/07)
2/5Mystique 15 (2004/07)
3/5Mystique 16 (2004/08)
4/5Mystique 17 (2004/09)
5/5Mystique 18 (2004/10)
Unnatural Instincts
1/3Exiles (I) 28 (2003/08)
2/3Exiles (I) 29 (2003/09)
3/3Exiles (I) 30 (2003/09)
Unnatural Selection
1/5Amazing Fantasy (II) 16 (2006/02)
2/5Amazing Fantasy (II) 17 (2006/03)
3/5Amazing Fantasy (II) 18 (2006/04)
4/5Amazing Fantasy (II) 19 (2006/05)
5/5Amazing Fantasy (II) 20 (2006/06)
Unredeemed
1/3New Excalibur 13 (2007/01)
2/3New Excalibur 14 (2007/02)
3/3New Excalibur 15 (2007/03)
Unscheduled Stop
The Unspeakable Shrine
1/2Savage Tales (I) 7 (1974/11)
2/2Savage Tales (I) 8 (1975/01)
The Unspoken
1/5Mighty Avengers 27 (2009/09)
2/5Mighty Avengers 28 (2009/10)
3/5Mighty Avengers 29 (2009/11)
4/5Mighty Avengers 30 (2009/12)
5/5Mighty Avengers 31 (2010/01)
Unstoppable!
Unthinkable
Untold Tales of the Marvel Universe
What if? (I) 23 (1980/10)
What if? (I) 24 (1980/12)
What if? (I) 25 (1981/02)
What if? (I) 26 (1981/04)
What if? (I) 27 (1981/06)
What if? (I) 28 (1981/08)
What if? (I) 29 (1981/10)
What if? (I) 30 (1981/12)
What if? (I) 35 (1982/10)
Untold Tales of the New Universe
Unusual suspects
1/4Daredevil/Spider-Man 1 (2001/01)
2/4Daredevil/Spider-Man 2 (2001/02)
3/4Daredevil/Spider-Man 3 (2001/03)
4/4Daredevil/Spider-Man 4 (2001/04)
Up is Down and Black is White
1/6Punisher (VII) 19 (2005/06)
2/6Punisher (VII) 20 (2005/06)
3/6Punisher (VII) 21 (2005/07)
4/6Punisher (VII) 22 (2005/08)
5/6Punisher (VII) 23 (2005/09)
6/6Punisher (VII) 24 (2005/10)
Up North and in the Green
1/2Exiles (I) 5 (2001/12)
2/2Exiles (I) 6 (2002/01)
Urban Jungle
1/4Ka-Zar (III) 8 (1997/12)
2/4Ka-Zar (III) 9 (1998/01)
3/4Ka-Zar (III) 10 (1998/02)
4/4Ka-Zar (III) 11 (1998/03)
Utopia