The List
There are 183 5 letter words with U as the first letter. The highest scoring is UNZIP at 16 points in Scrabble, though Words With Friends prefers UPJET at 18. 52 carry three or more vowels, which makes them useful for dumping a vowel-clogged rack.
UDALSUDDERUDONSUGALIUGGEDUHLANUHURUUKASEULAMAULANSULCERULEMAULMINULNADULNAEULNARULNASULPANULTRAULVASULYIEULZIEUMAMIUMBELUMBERUMBLEUMBOSUMBRAUMBREUMIACUMIAKUMIAQUMMAHUMMASUMMEDUMPEDUMPIEUMPTYUMRAHUMRASUNAISUNAPTUNARMUNARYUNAUSUNBAGUNBANUNBARUNBEDUNBIDUNBOXUNCAPUNCESUNCIAUNCLEUNCOSUNCOYUNCUSUNCUTUNDAMUNDEEUNDERUNDIDUNDUEUNDUGUNETHUNFEDUNFITUNFIXUNGAGUNGETUNGODUNGOTUNGUMUNHATUNHIPUNIFYUNIONUNITEUNITSUNITYUNJAMUNKEDUNKETUNKIDUNLAWUNLAYUNLEDUNLETUNLIDUNLITUNMANUNMETUNMEWUNMIXUNPAYUNPEGUNPENUNPINUNREDUNRIDUNRIGUNRIPUNSAYUNSETUNSEWUNSEXUNSODUNTAXUNTIEUNTILUNTINUNWEDUNWETUNWITUNWONUNZIPUPBOWUPBYEUPDOSUPDRYUPENDUPJETUPLAYUPLEDUPLITUPPEDUPPERUPRANUPRUNUPSEEUPSETUPSEYUPTAKUPTERUPTIEURAEIURALIURAOSURAREURARIURASEURATEURBANURBIAURDEEUREALUREASUREDOUREICURENAURENTURGEDURGERURGESURIALURINEURITEURMANURNALURNEDURPEDURSAEURSIDURSONURUBUURVASUSAGEUSERSUSHERUSINGUSNEAUSQUEUSUALUSUREUSURPUSURYUTERIUTILEUTTERUVEALUVEASUVULA