Familial predisposition for herniation of a lumbar disc in patients who are less than twenty-one years old. The parents of sixty-three patients who were less than twenty-one years old and who had operatively confirmed herniation of a lumbar disc were interviewed regarding a history of sever back pain, sciatica, and herniated disc, to determine whether aggregation of herniation of a lumbar disc occurs in families of patients in this young age-group. The parents of sixty-three additional patients who had a non-spinal orthopaedic diagnosis (control group) were matched for age and sex with the study group and were given the same interview. Of the patients who had herniation of a lumbar disc and were less than twenty-one years old, 32 per cent had a positive family history for that lesion compared with 7 per cent of the control group. The relative risk of development of herniation of a lumbar disc before the age of twenty-one years is estimated to be approximately five times greater in patients who have a positive family history. The results indicate a familial basis for herniation of a lumbar disc in patients who are less than twenty-one years old. 