The radiographs of 206 knees from 174 consecutive patients were reported blinded regarding joint space narrowing. Minimum joint space was measured using a millimeter ruler on weightbearing posteroanterior and skyline patellofemoral radiographs. Lateral radiographs were reported solely in terms of presence or absence of patellofemoral arthritis. The results showed objective joint space narrowing to less than 3 mm in the patellofemoral compartment of 32.7% of men and 36.1% of women older than 60 years. This high prevalence of patellofemoral disease in men has not been documented before. Arthritic changes occurred in the patellofemoral compartment in isolation in 13.6% of women and 15.4% of men older than 60 years. The lateral radiograph had poor results for detection of patellofemoral osteoarthritis with a sensitivity of 66% and specificity of 83%. The positive predictive value of an abnormal lateral radiograph was 52%. If requests for skyline radiographs had been confined to patients with abnormal patellofemoral joints as assessed on lateral films then 28 normal joints would have been imaged and 14 abnormal joints would have been missed. Osteoarthritis is more common in the patellofemoral joints of men and women than previously documented and only can be properly assessed using a skyline radiograph.