A number of factors were identified as causes of mortality in 254 (59%) of 431 sea ducks submitted for necropsy at the USGS-National Wildlife Health Center, Madison, Wisconsin from 1975 until 2003. Bacteria causing large outbreaks of mortality were Pasteurella multocida and Clostridium botulinum Type E. Starvation was responsible for large mortality events as well as sporadic deaths of individuals. Lead toxicity, gunshot and exposure to petroleum were important anthropogenic factors. Other factors that caused mortality were avian pox virus, bacteria (Clostridium botulinum Type C, Riemerella anatipestifer and Clostridium perfringens), fungi (Aspergillus fumigatus and an unidentified fungus), protozoans (unidentified coccidia), nematodes (Eustrongylides spp.), trematodes (Sphaeridiotrema globulus and Schistosoma spp.), acanthocephalans (Polymorphus spp.), predation, cyanide and trauma (probably due to collisions). There were also a number of novel infectious organisms in free-living sea ducks in North America, which were incidental to the death, including avipoxvirus and reovirus, bacteria Mycobacterium avium, protozoans Sarcocystis sp. and nematodes Streptocara sp. Apart from anthropogenic factors, the other important mortality factors listed here have not been studied as possible causes for the decline of sea ducks in North America.