We have developed an automatic system for the measurement of joint space in the knee using computerized analysis of digital stored images of knee radiographs. PA X-rays of the knee are positioned on a conventional viewing box. A video image of the radiograph is converted to digital form, stored and displayed on a closed circuit television monitor. This is edited and analysed by a microcomputer. After careful positioning under a computer generated grid on the television screen, automatic measurements are made using an edge detection facility. Two different edge detection algorithms were compared. Reproducibility is very good. Inter- and intra-observer relationships are also good with no significant difference between observers using a paired t-test and very good correlations. Results show that rapid, reliable and accurate measurement of joint space size can be achieved using digital image analysis. The application of image handling computers to radiological scoring is important in our understanding of joint destruction and the progression of the rheumatic diseases.