We studied the influence of the radiographic procedure and joint positioning on knee joint space width (JSW) in 10 healthy volunteers, and the intrareader reproducibility of JSW measurements on radiographs performed 2 weeks apart using a standardized procedure. Results show that a 5 or 10 downward inclination of the X-ray beam and 15 or 30 of induced external foot rotation significantly reduced JSW. In contrast, knee flexion increased JSW. The mean differences and S.D. in the measurement of JSW between two sets of radiographs taken 2 weeks apart were not statistically significant, ranging from -0.07 mm (S.D. 0.38) to 0.020 mm (S.D. 0.38). Our findings indicate that modifications in knee flexion, foot rotation and X-ray beam inclination influence JSW. Therefore, standardization of joint positioning and of the radiographic procedure is necessary to obtain comparable radiographic images on successive X-rays.