Adult onset Still's disease is recognized as an adult variant of the systemic form of juvenile rheumatoid arthritis, whose disease-predisposition is still debated. On the other hand, the association between HLA subtypes and several groups of seronegative arthritis including psoriatic arthritis has been well documented. This report describes a family where adult onset Still's disease in a young man and psoriatic arthritis in his father were seen. Both patients were HLA-B39-positive, which was likely playing important pathogenic roles in the latter case. Clinical and immunological aspects of HLA-B39-related inflammatory diseases are also discussed.