Radionuclide bone scanning is a sensitive detector of new bone formation. It offers the possibility of more complete description of the distribution of hypertrophic pulmonary osteoarthropathy. The scintigraphic findings in 48 confirmed cases of hypertrophic pulmonary osteoarthropathy (HPO) are described. Several new observations about the involvement of bones and distribution pattern have been made: the skull (mandible and/or maxillae) was involved in 42% of cases, the scapulae in 67%, and the patellae in 50%. Clavicles were involved in 33% of cases. With the exception of the humerus, the proximal and distal portions of each long bone were involved with equal frequency. Involvement of the extremities was invariably present, and disease almost always appeared more active in the lower than in the upper extremities. Asymmetric involvement of the extremities was noted in 17% of cases, and there was irregular involvement in 15%. Differentiation of metastatic disease from HPO by bone scanning is generally not difficult, since the most frequent patterns of distribution of these two conditions are entirely different.