Plain radiographs of the urinary tract on 2707 normal volunteers were reviewed to establish the prevalence and patterns of spina bifida occulta in the normal population. Abnormalities in the formation of the posterior arch were identified in 23%, and four different patterns are described. In this group of subjects the condition was twice as common in males as in females (P less than 0.001) and commoner in younger adults. The commonest defects were either in S1 alone or involved S1 and S2 together. Isolated defects at L5 or S2 were rare. The level of opening of the posterior sacral arcs was assessed and a high level of opening was shown to correlate with the presence of spina bifida occulta.