CT findings on 170 patients presenting with isolated moderate to severe neurodevelopmental delay have been compared with the final diagnosis. MRI was undertaken in 29 patients. Eighty per cent of the patients remained undiagnosed, and although the MRI findings were abnormal in 65.5% compared with only 30% of the CT examinations, imaging uncommonly suggested a specific diagnosis. Biochemical and chromosomal investigations were significantly more diagnostic. The results of these studies should restrict the number of non-contributory neuroradiological examinations. The superior intrinsic contrast of MRI will detect more lesions, particularly in white matter, but these are rarely diagnostic and where access to MRI is limited, CT is usually adequate.