Failure of fusion of the posterior arches of the lumbosacral spine above S3 was sought on frontal radiographs of 653 patients attending an accident and emergency (A & E) department. The patients were aged from 2 months to 98 years and represent all those with relevant information discharged as A & E outpatients over a 2-year period. Presenting complaints of backache or enuresis and inadequate radiographs were excluded. Spina bifida occulta was diagnosed in 22% of the whole group. The incidence was much higher in those below the age of 40 years (29.2%) compared with those above (9.8%). The overall age-adjusted incidence was 17.3%. Our study is an attempt to judge the ‘off-the-street’ prevalence of spina bifida occulta. It supports the notion that spina bifida occulta is a common anomaly, of no clinical significance on its own.