Congenital anorectal anomalies present with a wide spectrum of anatomical deformity. The level of atresia, the extent of the development of the pelvic musculature, and the presence of associated anomalies all greatly influence treatment options. Further complexity may be introduced by previous attempts at correction. Magnetic resonance imaging (MRI) recently has been suggested as an effective tool in evaluation of such patients for surgical repair. Thirteen patients undergoing evaluation for either primary or secondary operations for imperforate anus were studied with MRI. Age range was newborn to 26 years. These patients had anorectal deformities in various stages of treatment--from newly diagnosed imperforate anus in the newborn to patients who had undergone previous surgical repair of their deformity with poor results. In newborns, MRI was very useful in assessing the level of atresia and determining whether perineal repair was advisable. In older patients with high atresias, MRI was useful in planning operative strategy and predicting the outcome by providing information about the pelvic musculature. This information was directly related to the size of the patient, ie, the larger, and therefore the older the patient, the more detail MRI provided about the pelvic musculature. Especially in secondary cases, this information proved to be helpful in determining the cause of the initial failure, ie, lack of adequate pelvic musculature or poor placement of the distal colon in relation to the striated muscle complex at the initial operation. MRI demonstrated previous unsuspected lesions such as tethered cord, sacral deformity or urinary tract abnormalities.(ABSTRACT TRUNCATED AT 250 WORDS)