The pattern of joint involvement on conventional radiographs and the four hour gammacamera image of 99Tc HMDP bone scans were studied in 33 patients with generalised nodal osteoarthritis. Both techniques showed the predominant involvement of the distal interphalangeal, scaphotrapezial, and first carpometacarpal joints. Some joints were abnormal just on one investigation: either x ray or scan alone. Others showed a marked dissimilarity in the severity of involvement on x ray compared with scan. This discrepancy between x ray and scan suggests that the scan is imaging a different process than the radiograph and offers a different way of assessing change in osteoarthritis.