ABSTRACTA two‐year‐old Scottish terrier was referred for investigation of a chronic skin problem. A diagnosis of generalised demodectic mange with a deep pyoderma was made and treatment commenced with trimethoprirn‐sulphamethoxazole and amitraz washes. On the sixth week of treatment the owner reported that the dog had developed haematuria. Investigations revealed that the dog was thrombocytopenic and anaemic. Antimicrobial treatment was stopped and the platelet count returned to normal over a three week period. A diagnosis of trimethoprim‐sulphamethoxazole associated thrombocytopenia w