We analyzed the incidence and cause of glenohumeral osteoarthritis including its early stage with plain radiographs. In Kishiwada City Hospital we treated 4035 patients with orthopaedic complications in 1995. A total of 345 patients (390 shoulders) had shoulder diseases. The patients with primary glenohumeral osteoarthritis were 2 men and 14 women (18 shoulders) aged 60 to 86 years (mean 71.9 years), and 2 cases were secondary osteoarthritis. In 2 cases (3 shoulders) of severe osteoarthritis other joints were also affected. Only 4 (25%) of the patients with primary glenohumeral osteoarthritis had osteoarthritic changes only in the glenohumeral joint. The incidence of primary glenohumeral osteoarthritis was 0.4% in the patients with orthopaedic complications and 4.6% in the patients who had shoulder diseases. Women were more likely to have primary glenohumeral osteoarthritis than men, and patients older than 60 years of age were more likely to have it than younger patients.