In the period between 1981 and 1983, we treated 40 patients who had osteochondritis dissecans of the knee. Seventeen patients were skeletally mature and had large lesions (2 to 3 cm in diameter) of the medial femoral condyle that required surgery. Each of these patients, who underwent antegrade curettement, bone grafting, and pinning, had either failed a conservative program or had a loose or separated fragment. The average age at the time of surgery was 17. Followup ranged from 5 to 7 years. Postoperative evaluation included the criteria of the Hughston et al. rating scale, subjective assessment, clinical examination, and AP, lateral, and tunnel radiographs. According to the osteochondritis dissecans rating scale described by Hughston et al., there were two excellent, nine good, five fair, no poor results, and one failure. Additional evaluation, not included in the Hughston rating scale, consisted of Cybex II muscle strength testing, strict grading of preoperative and postoperative roentgenograms for degenerative changes, and magnetic resonance imaging. Roentgenograms demonstrated that 16 of the 17 lesions healed. The average healing time was 8 months. Preoperative degenerative changes occurred frequently; postoperatively, there was some progression of these changes in the majority of cases. Magnetic resonance imaging demonstrated that the articular surface was smooth in 57% of the knees and in 43% there was some degree of irregularity. Although the integrity of the articular cartilage could be documented, magnetic resonance imaging is of questionable value in the postoperative evaluation of this condition, because the status of the articular cartilage could not be correlated with the degree of degenerative changes or results.(ABSTRACT TRUNCATED AT 250 WORDS)