Recent reports have suggested an association between congenital anorectal anomalies and occult spinal dysraphism. Eighty-seven patients with anal agenesis have been treated at this institution over the last 14 years. Two of these patients had spinal cord anomalies recognized at birth (a myelomeningocele and a tethered spinal cord). Two additional patients presented with progressive neurologic deficits in early childhood and were each found to have a tethered spinal cord. To further assess the magnitude of this problem, we have used magnetic resonance imaging (MRI) of the spine to survey prospectively 23 infants with anal agenesis. Twenty-one former patients who were asymptomatic were recalled and also studied. Four of 44 patients (9%) were found to have significant occult spinal dysraphism; each child had undergone neurosurgical operation without morbidity. MRI found each child to have a tethered spinal cord, either as an isolated lesion (2) or in association with a syrinx (1) or lipomyelomeningocele (1). One of these patients had a neurologic deficit detected on careful preoperative evaluation. The other three, two of whom were less than 2 years old, had no detectable deficit. Neither the extent of the anorectal malformation, the absence of associated congenital anomalies, nor the demonstration of normal vertebral anatomy on plain radiographs of the spine precluded the presence of occult spinal dysraphism. Therefore, we recommend that all patients with anorectal anomalies undergo MRI imaging of their spines during initial evaluation to screen for occult spinal dysraphism. In addition, consideration should be given to recalling older patients for MRI evaluation.