The intra-osseous pressure, PO2, and PCO2 were measured in 32 hips (21 patients) which were painful but showed no severe degenerative changes. Pre-operative scintigraphy and radiography was performed in all patients. Thirteen hips showed early osteoarthritis, eight had early osteonecrosis, and 11 had no changes. Core biopsies were performed and the bone was examined histologically and graded for necrosis. Histologically, necrosis was present in 27 specimens. Scintigraphic findings did not correlate with the histological results but were more closely related to the radiographic findings. The intra-osseous pressure in hips with histological necrosis (mean 47 mmHg) was significantly higher than in hips without necrosis (mean 26 mmHg). The PO2 was lower in bone with histological necrosis (mean 44 mmHg) than in bone without (mean 71 mmHg). PO2 increased and intra-osseous pressure decreased after decompression. The results confirm that ischaemia plays a central role in the development of necrotic changes in bone. Histological necrosis was found in hips with radiographic signs of osteonecrosis and in those with osteoarthritis. Radiography, and scintigraphy are shown to be insensitive methods for differentiating between those disorders.