Summarized are necropsy results from 222 carcasses of Common Loons (Gavia immer) submitted to the National Wildlife Health Research Center from 1976 through 1991.  The carcasses were from 18 states, and 10 or more birds each were from Minnesota, Florida, Virginia, Michigan, Wisconsin, Maine, and North Carolina.  Seventy-three (33%) carcasses were emaciated, and in some of these birds emaciation was thought to be related to exposure to mercury.  OVer 40% of these emaciated birds were from Florida.  Trauma, including blunt trauma of unknown origin, outboard motor propeller wounds, and shooting caused the deaths of 49 (22%) loons, 30 of which were from Minnesota.  Diseases, primarily avian botulism type E and aspergillosis, account for 39 (18%) moralities and lead poisoning for 14 (6%), 11 of which had fishing sinkers in their stomachs.  Most of the avian botulism type E cases occurred during two outbreaks on Lake Michigan.  Seven of the 14 lead-poisoned birds were from Minnesota.  Nine (4%) birds died of miscellaneous causes and 9 (4%) of drowning, primarily from entanglement in nets.  No diagnosis could be reached for 29 (13%) carcasses.  Sample bias precludes interpretation of these data to represent actual proportional causes of mortality in the loon population.  However, the sample size is sufficient to clearly identify major causes of mortality.