In a Finnish kindred consisting of 192 descendants from two marriages of a male ancestor born in 1868, the lumbar spines of 105 of the 170 living members were X-rayed. Spondylolysis was found in 22 individuals. In addition, six of them had spondylolisthesis, four had spina bifida occulta, and two had a transitional lumbar/sacral vertebra. Seven members of the kindred without spondylolysis had spina bifida occulta and 10 had transitional lumbar vertebrae. The pedigree is consistent with autosomal dominant inheritance and incomplete (about 75%) penetrance for spondylolysis. It raises the question of a common aetiology for several congenital disturbances in the formation of lumbar vertebrae and possibly supports the concept of variable expressivity of a "spondylolysis gene".