We studied the normal radiographic anatomy of the coccyx in 120 asymptomatic subjects and performed a retrospective review of the results in fifty-one patients who had had a partial or total coccygectomy for idiopathic coccygodynia during a twenty-year period. Of the asymptomatic subjects, the sacrococcygeal joint was fused in forty-four (37 per cent); the first intercoccygeal joint, in twelve (10 per cent); and the second intercoccygeal joint, in fifty-two (43 per cent). Four types of configuration of the coccyx were identified on the lateral radiographs. In Type I the coccyx was curved slightly forward, whereas in Type II the curve of the coccyx, which pointed straight forward, was more marked. In Type III the coccyx was angulated forward sharply, and in Type IV it was subluxated at the sacrococcygeal or the intercoccygeal joint. Most subjects (68 per cent) had a Type-I configuration. Of the fifty-one patients with idiopathic coccygodynia, twenty-six (51 per cent) showed fusion of the sacrococcygeal joint; six (12 per cent), of the first intercoccygeal joint; and twenty-five (49 per cent), of the second intercoccygeal joint. In most patients (69 per cent) the coccyx had a Type-II, III, or IV configuration. Thirty-one patients had undergone a partial coccygectomy and twelve, a total coccygectomy; in the remaining eight patients the extent of the coccygectomy could not be determined. The results of surgery were excellent or good in thirty-two (88 per cent) of the thirty-six patients who were followed for at least two years.(ABSTRACT TRUNCATED AT 250 WORDS)