Thirty-nine temporomandibular joint autopsy specimens were examined by microscopy and tomography for erosive changes. We found two types of erosive changes, an extensive type with complete loss of cartilage and a local type with retained articular cartilage. On microscopic examination nearly twice as many temporal components as condyles were eroded. The erosions were generally more extensive in the condyle. Erosions in the condyle were evenly distributed. In the temporal component there was a slight predominance of erosions located to the lateral part of the tubercle. The radiologic investigation underestimated both the presence and the extent of the erosions. Positive predictive values and negative predictive values were 0.70 and 0.83, respectively, for erosions in the condyle and 0.91 and 0.68 for erosions in the temporal component. It is suggested that the initial event in osteoarthrosis of the TMJ can occur as a subarticular hard-tissue change. The need for more accurate diagnostic tools than radiography should be stressed.