In a triple-blind controlled study, significant pain alleviation (p less than 0.01) was registered in patients with osteoarthrosis (OA) of the hip in the first 24 h after hip joint distension. There was no significant difference between treatment with indoprofen (a new non-steroidal anti-inflammatory drug for intra-articular use) and placebo. Pain relief lasted for at least 12 weeks in approximately half of the patients, in whom a significant increase in joint mobility was registered. Intra-articular indoprofen was not superior to placebo as regards pain relief or increased joint mobility. These findings indicate that the effect of a single hip joint distension in OA is due to a mechanical distension of the joint capsule, perhaps involving disruption of local agglutinations, and not due to suppression of inflammation.