Thirty-five of 50 patients with different hip joint disease had sonographic evidence of joint effusion. Arthrocentesis confirmed effusions in 30 of these 35 patients. Thirty-two of the 35 patients had nocturnal pain. Both nocturnal pain and sonographic evidence of effusion decreased after aspiration (15 patients) and aspiration and injection of corticosteroids (15 patients). In a further group of 61 patients who subsequently had Charnley arthroplasties, 35 had positive sonograms before operation. Of these, 25 had effusions confirmed at operation, the remaining 10 having synovitis and capsule thickening. Again a correlation was found with nocturnal pain. The sensitivity of sonography in detecting hip joint effusion was 92% with a specificity of 70%. Nocturnal pain had a lower sensitivity, 85%, but higher specificity, 94%.