The Meckel syndrome comprises a variety of defects including the classical triad of occipital encephalocele, cystic kidneys, and polydactyly. The frequencies of the various defects are more accurately represented in the affected sibs of probands than in the probands themselves, since the latter are selected according to severity and preconceived notions of what constitutes the syndrome. In a series of 38 such sibs, all had cystic dysplasia of the kidney, 63% had an occipital meningocele, 55% had polydactyly, and 18% had no reported brain malformation. In families in which the proband had the classical triad, only 68% of the affected sibs had it. It is concluded that the diagnosis of Meckel syndrome may not be valid in the absence of cystic kidney dysplasia. In babies with encephalocele or anencephaly, pathologic examination, particularly of the kidneys, is important in determining risk of recurrence. This approach to estimating the variability of a syndrome might profitably be extended to other genetically determined pleiotropic conditions.