In a double-blind between-patient study the efficacy of diclofenac potassium, a non-steroidal anti-inflammatory drug, was assessed in 93 patients with mild to severe sprained ankles; patients with more severe sports injuries were excluded. Patients were randomly allocated to receive 50 mg diclofenac potassium three times daily, 20 mg/day piroxicam or placebo for 7 days. Diclofenac potassium was more effective than piroxicam or placebo in reducing pain at rest and on walking, but did not significantly reduce the degree of swelling when measured volumetrically by water displacement. No serious side-effects were reported. It is concluded that diclofenac potassium is useful in the treatment of moderately inflammatory processes with the advantage that it had a rapid onset of action with good overall tolerability.