Six women and three males, who initially had seronegative monarticular arthritis of the manubriosternal joint (MSJ) were followed for a mean period of 7.1 years (range 0.8-17 years). Rather severe, progressive disease occurred in five patients, three of whom had pustulosis palmoplantaris (PPP) and one acne vulgaris. The course was milder in four patients who had either psoriasis vulgaris, psoriasis in the family or HLA-B27, and the disease remained localized to the MSJ in these patients. None of the patients developed sacroiliitis or peripheral erosive joint changes, but one patient with PPP developed paravertebral ossification.