In an open, controlled, multi-centre clinical field trial, seven 'naturally occurring' outbreaks of acute febrile (rectal temperature greater than or equal to 39.5 degrees C) respiratory disease in housed calves were treated with a single antimicrobial agent, and either the non-steroidal anti-inflammatory drug (NSAID) carprofen (n=95) or flunixin meglumine (n=92) on an alternate basis. Carprofen was administered as a single subcutaneous injection at a mean dosage of 1.4 mg kg(-1) (range 1.2 to 1.9 mg kg(-1)) body weight on the first day and flunixin meglumine by intravenous injection at a mean dosage of 2.0 mg kg(-1) (range 1.2 to 2.6 mg kg(-1)) body weight on the first 3 consecutive days. All calves were examined clinically immediately prior to initial treatment and on three occasions up to 1 week after the end of treatment. There were no statistically significant differences between NSAID groups in reduction of clinical parameters between examinations, or in overall efficacy. This trial demonstrated that a single dose of carprofen was equally effective as three daily doses of flunixin meglumine as adjunctive therapy to antimicrobial treatment in acute respiratory disease in calves.