Two adult Wied's marmosets (Callithrix kuhlii) presented with jaundice, anemia, and weight loss. Death of one individual was attributed to renal tubular necrosis; liver and kidney were positive for Leptospira antigen by immunohistochemical (IHC) staining. The second animal was negative for antigen by IHC staining, but serologically positive for Leptospira borgpetersenii serovar ballum with an eightfold titer increase in paired samples, and was euthanized because of unresponsiveness to treatment. Environmental contamination by mice was suspected as the Leptospira source.