Joint surface area (JSA) and mean joint space width (MJSW) at the hip were measured using a ICMS-Techline computer program to analyze digitalised frontal weight-bearing roentgenograms of the pelvis. With this technique, the portion of joint space studied is always the same in a given patient and is enclosed within an acute ECS angle whose apex C is the center of the head of the femur and whose ends E and S are the lateral rim of the acetabulum and the highest point of the homolateral sacral wing respectively. ECS varies across individuals but remains constant in a given hip. Twenty hips were included in the first part of the study. For each hip, three roentgenograms were taken at five-minute intervals by three different radiologists who used their own constants (settings, position of the subject). JSA and MJSW are determined five times on each film by two different observers who had no information on the films under study. The interobserver coefficient of variation (CV) was 4.7% for JSA and 3.3% for MJSW. Intra-observer CVs were 2.97 and 3.54% for MJSW and 4.32% and 5.13% for JSA. There was a very close correlation between MJSW and JSA (r = 0.87, p < 0.0001). MJSW was then determined for roentgenograms of 30 hips with osteoarthritis. Results were compared with the values obtained using Lequesne's method of joint space measurement at the site of maximum narrowing. Measurements were performed in a double-blind fashion by two observers. The correlation coefficient was r = 0.89 (p < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)