Acquired coccygeal nodule due to repeated stimulation by a bicycle saddle.

Four Japanese male students, aged 14, 16, 18 and 19, were found to have a nodule in their coccygeal regions. The typical lesion was a rugby-ball-shaped, normal-colored or slightly erythematous nodule measuring 5 to 6 cm in its long axis. Histopathologically, it showed hyperplasia of collagen fibers and slight acanthosis. All four students rode to school by bicycle over long distances and long periods; three of them developed the nodule after they started bicycle riding to school. We found that the nodule shape corresponded to the saddle of the bicycle. X-ray abnormalities included bending or anterior dislocation of the coccyx in all cases and spina bifida in two cases. These nodules may have developed as a result of irritation of this region by the saddle of the bicycle.