To study the impact of articular hypermobility on the clinical and radiological features of hand osteoarthritis (OA) and to investigate whether hand osteoarthritis associated with hypermobility should be considered a separate subset of hand OA. Fifty consecutive female patients with clinical hand OA and thumb base symptoms were examined for hypermobility according to the Beighton criteria. Thirty one of the 50 patients had hypermobility features (Beighton score > or = 2) and 17 patients fulfilled four or more Beighton criteria. Corresponding figures for 94 control patients were 30 (p < 0.05) and nine (p < 0.001) respectively. Patients with hypermobility features were characterised clinically and radiologically by fewer and less severely involved interphalangeal joints. Radiologically, two fairly distinct subsets could be identified: Severe interphalangeal OA in which the prevalence of hypermobility was similar to controls, and patients with predominant involvement of the first carpometacarpal joint (CMC 1), most of whom had evidence of hypermobility. A causal relation exists between articular hypermobility and development of thumb base OA, and hypermobility associated hand OA constitutes a definite clinical and radiological subset of hand OA. In the clinical setting, the easily applied hypermobility criterion of passive dorsiflexion of the fifth finger > or = 90 degrees is useful in identifying most patients with hand OA and hypermobility.