Medical records and necropsy reports of 410 pigeons and doves from the Zoological Society of San Diego were examined to determine if birds treated with fenbendazole (FBZ) or albendazole (ABZ) had a higher incidence of toxic-related signs than untreated control birds. Birds presented for nonmedical reasons (quarantine, relocation, preshipment examinations, and routine parasite screening) were used exclusively to avoid confounding variables. Birds were observed for 45 days after presentation. Compared with a 4–6% mean weight gain in control birds during the treatment period or hospitalization, birds treated with FBZ had a mean weight loss of 16.1% (P < .001), and birds treated with ABZ had a mean weight loss of 13.3% (P = .004). The percentage of birds with marked leukopenia (white blood cell < 1000/μ l) was higher in FBZ-treated birds (87.5%, P < .001) and ABZ-treated birds (100%, P = .013) than in control birds (12.5%). Bone marrow hypoplasia was found in FBZ-treated birds (48.6%, P < .001) and ABZ-treated birds (83.3%, P < .00l) but not in control birds (0%). Small intestinal crypt epithelial changes were not found in control birds but were identified in 35.1% of FBZ-treated birds (P < .001) and in 58.3% of ABZ-treated birds (P < .001). Percent survival was lower in FBZ-treated birds (55.9%, P < .001) and ABZ-treated birds (66.7%, P < .001) than in control birds (90%). Of the birds that died during observation, those treated with FBZ (mean 13.9 days, P < .001) and ABZ (mean 9.3 days, P < .001) had shorter average survival times after presentation than did control birds (mean 29.8 days). Fenbendazole effects appear to be dose related. Birds treated with 100 mg/kg of FBZ showed significantly greater mean weight loss (18.8%) and lower survival (14.8%) than did birds treated with 50 mg/kg of FBZ, with 8.3% weight loss (P = .005) and 75.4% survival (P < .00l). Of the birds that died during observation, those treated with 100 mg/kg of FBZ had shorter mean survival times after treatment (7.9 days) than did birds treated with 50 mg/kg of FBZ (23.9 days, P < .001). These findings are consistent with a toxic etiology and suggest that birds of the order Columbiformes are susceptible to toxicosis after FBZ or ABZ administration.