Metastatic carcinoma of probable transitional cell origin was identified histologically in 66 of 360 (18%) sub-adult and adult California sea lions examined at post mortem at The Marine Mammal Center following live stranding in central California between January 1981 and December 1995. Affected animals presented with perineal edema, paresis or paralysis of the hind flippers and often prolapse of the penis if male. They were euthanized or died within 3 days of stranding, and on post mortem examination had characteristic Gaseous, severely enlarged sub-lumbar lymph nodes. These often encompassed the ureters, resulting in hydroureter and hydronephrosis, and in some cases invaded the sub lumbar musculature and adjacent vertebrae. Widespread metastases were common, involving omentum, spleen, liver, lymph nodes and lungs. On histology, the tumor masses showed a mixed pattern of squamous and adenomatous differentiation typical of transitional cell carcinoma. Immunohistochemical stains confirmed the neoplastic cells as epithelial, but did not identify them further as transitional in origin. PCB and DDT levels in blubber from affected animals were significantly higher (> 3 fold) than levels of these contaminants in blubber of sympatric California sea lions dying from acute trauma (e.g. gunshot). However, blubber thickness in all animals had a significant effect on contaminant levels, as did sex (thinner animals and females had higher contaminant levels). Traces of DNA abducts indicative of exposure to polyaromatic hydrocarbons (PAM) were detected in sea lions with carcinoma, but the profile of these adducts was not similar to that of abducts in marine fish exposed to PAHs.