The gradient in prevalence of idiopathic osteoarthritis of the metacarpophalangeal (MCP) joints from the index to the little finger was used to test three theories. Trauma was correlated by analysis of fracture incidence in the four rays in 142 patients. Correlation with utilisation of arc was assessed by goniometric measurement of mean MCP flexion in five hand positions in a sample of 200 activities, and recording utilisation of these positions by behavioural observation techniques in the course of 2831 actions, 2746 domestic and 85 nondomestic. Both correlations were negative. The hypotheses of excessive loading and incomplete use were compared and tested against external evidence. It is suggested that the gradient in MCP prevalence of osteoarthritis is consistent with a bifactorial aetiology, with site and incidence determined by incomplete use, and with the rate of progress by cartilage loading.