The siblings and parents of 40 patients (cases) with lumbar disc herniation (aged 18 years or younger) who underwent surgery at the Department of Orthopaedics at Toyama Medical and Pharmaceutical University Hospital between 1976 and 1990 were examined for incidence of lumbar disc herniation. A total of 120 randomly sampled, age- and sex-matched patients (controls) who were hospitalized during the same period due to extraspinal diseases also were studied in the same manner. A survey of the occurrence of definite lumbar disc herniation was carried out that included 75,237 students in elementary, junior high, and senior high schools in Toyama Prefecture, Japan, covering a mean period of 3 years and 4 months. The incidence of surgically treated lumbar disc herniation among people aged 18 years or younger was calculated, and the expected value of disc herniation was obtained in an age-specific manner, on the basis of the age distribution of encumbrances in the above case-control study. The encumbrances of 18-year-old or younger patients with lumbar disc herniation showed familial predisposition, with an odds ratio of 5.61 in comparison to the controls. It was suggested that there is familial clustering of lumbar disc herniation among the encumbrances of 18-year-old or younger patients with lumbar disc herniation.