We report a cytogenetic study of 200 children with mental retardation and three or more major or minor congenital anomalies. In all cases, the chromosomes were studied with conventional staining methods (nonbanding) and with at least one of the following techniques: Q, G, or R banding. In a few patients, C banding and in vitro differentiation with BUDR and acridine orange (R banding) were also used. In patients with structural abnormality, parental chromosomes were studied using the same techniques. A chromosomes abnormality was found in 42 patients (21%). Of these, 16(8%) had complete or mosaic aneuploidies (11 autosomal and 5 gonosomal); and 26 (13%) had structural defects. In 21 of the latter the structural abnormality occurred as a de novo rearrangement, and in 5 the defect was inherited from a parent carrier of a balanced rearrangement. The contribution of chromosome aberrations to the cause of (idiopathic) MCA/MR syndromes is discussed.