Radiographic joint space narrowing is the hallmark of progression in osteoarthritis (OA). An accurate measure of progression for OA requires a precise quantitative method. New techniques have been designed to quantitatively assess knee and hip joint space using computer analysis of digitally stored radiographs. The interobserver coefficient of variation of the technique ranges from 1% to 3.3% and is clearly better than that obtained by the use of dividers and rule. A precise standardization of the radiological procedure as well as use of films of good quality are required for a good reproducibility of the method. The computerized measurement of joint space size from standard radiographs is applicable to monitor progression of hip and knee OA, and appears to be of value in the evaluation of disease-modifying therapies for OA.