Eighty‐five mentally retarded patients had their chromosomes examined by using the high resolution banding techniques. Fifty‐nine of them had multiple congenital anomalies and/or dysmorphic features. Twenty‐six had mental retardation but no major anomalies; they were primarily suspected of having the fragile X syndrome. This suspicion was first excluded. Fifteen patients were found to have a chromosome aberration, interpreted as clinically significant. Eleven of them had been examined earlier by conventional methods. The aberrations found were divided into three groups: (I) those detectable with an accuracy of ca. 300–400 bands (N = 4), (II) those detectable with an accuracy of ca. 400–550 bands (N = 6) and (III) those detectable with an accuracy of ca. 550–850 bands (N = 5). The aberrations are described an