Three unrelated girls presented with a developmental syndrome of hypertrophy involving half or a quadrant of the body and not involving the face. The appearance was one of inappropriately large size of the affected side rather than contralateral atrophy. On the larger side, there was hypertrophy of muscle and increased power as well as an increase in diameter, but not in length, of long bones. There was areflexia and decreased pain and temperature sensation on that side. The patients also had progressive scoliosis and foot deformities on the enlarged side. One patient had a lumbar myelomeningocoele , and all 3 had a family history of neural tube closure defects. EMG, nerve conduction studies, EEG, skull x-rays, PEG, and cerebral CT scans were normal. Myelography did not demonstrate an enlarged cord, and in particular there was no evidence for syringomyelia. Chromosome studies revealed normal karyotypes. Sex chromatin was female on both sides in one patient. A defect of the dorsal lip of the neural tube or the neural crest is postulated to explain the abnormality. The association with closure defect in one patient and a positive family history of other neural tube defects in all 3 patients suggests that the developmental defect occurs at an early embryonic stage. Recognition of the syndrome is important. It can be distinguished clinically from hemiatrophy of cerebral origin. The neurological abnormalities are static, but the scoliosis is progressive and requires correction. The condition is associated with an increased prevalence of neural tube closure defects in the family, and forms part of a spectrum of genetically and embryologically related CNS malformations with multifactorial inheritance. Probands, parents, siblings and parents' siblings should be counselled that the risks of spina bifida and anencephaly in their offspring are the same as those in relatives of probands with classical neural tube defects, and should be offered prenatal diagnosis.