A randomised, double-blind placebo controlled study was undertaken to assess the analgesic efficacy of diclofenac. Following major abdominal surgery and 12 hours later, patients received either placebo or diclofenac 75 mg intramuscularly and their cumulative morphine requirements administered by a patient-controlled system over 24 hours were compared. Pain scores were also measured. Arterial blood gases and coagulation studies were assessed pre- and postoperatively. Morphine consumption was significantly greater in the placebo group (59.5 compared to 38.5 mg, p less than 0.01). Pain scores were significantly lower in the diclofenac group at 4 hours, but not thereafter. Arterial carbon dioxide was significantly increased in the control groups. There was no significant change in platelet count within each group, but a significant difference between the groups (p less than 0.05).