We examined the pelvic radiographs of two groups of patients (more than 12 years of age) from six medical centres. Hip dysplasia was considered to be present if Shenton's line was broken and more than one third of the femoral head was revealed to be uncovered in an antero-posterior radiograph of the pelvis. Patients with hip dysplasia due to teratological or neurological causes were excluded. There were 291 patients with treated or untreated hip dysplasia in the dysplastic group. The control group of 415 individuals was collected from consecutive outpatients (with a pre-set standardised female/male ratio) for whom an antero-posterior radiograph of the pelvis had been made in one of two medical centres and which did not disclose any abnormality of the hip joints. The aim of the study was to assess the coexistence of hip dysplasia and occult spinal dysraphism. Radiographs of all patients were examined, and any partial or complete defect of the posterior vertebral arch was recorded. In the dysplastic group, a defect was recorded in 23% (67/291) radiographs and in the control group in 12% (48/415). In both groups, L5 and S1 were the most commonly recorded sites with a defect. In the dysplastic group, a defect was recorded in 56/190 females and in the control group in 30/302 females. In males, there was no significant difference between the recorded findings in the two groups. In females with hip dysplasia, occult spinal dysraphism seems to be fairly common.