Osteoid osteomas located within the capsule of a joint are uncommon and present challenging diagnostic difficulties. Twelve patients with an intra-articular osteoid osteoma were studied retrospectively to determine the spectrum of clinical and radiological findings. The features differ significantly from the well known classical hallmarks of the extra-articular lesion. The symptoms are non-specific with pain similar to other common joint disorders and the response to salicylate less precise. On plain radiography intra-articular lesions are difficult to identify, lacking the florid periosteal new bone and intense perifocal sclerotic reaction. A detectable focal lesion is commonly absent with considerable delay between the onset of symptoms and radiological detection of the tumour nidus. Bone scintigraphy and computed tomography (CT) are essential for an accurate and early diagnosis. These imaging techniques reveal abnormalities in bone and cartilage growth, new bone formation and sclerosis distant from the tumour on either side of the joint, and disruption of the articular surface. Radiological follow up after surgery shows that these changes can be reversible, especially when the diagnosis is made early.