This presentation is designed to show the diagnostic implications of chromosomal abnormalities, and how in some cases chromosome analysis may be helpful in prognosis and counselling. Most males with Klinefelter's syndrome have chromatinpositive nuclei and an abnormal sex chromosome complex (usually XXY). In Turner's syndrome many such females have chromatin-negative nuclei and a deficient sex chromosome complex (usually XO). Multiple-X females have unusual chromatin patterns (two or three masses of sex chromatin) and abnormal sex chromosome complexes (XXX, XXXX, XO/XXX, etc.). One of the parents of a translocation mongol may carry a translocation chromosome and pass it to future children. Cytogenetic data are therefore essential for genetic counselling. Mosaic and deletion mongols may show incomplete manifestations of mongolism, which make diagnosis difficult; chromosome analysis is helpful in diagnosis, and in prognosis concerning mental development. Abnormal chromosome numbers result from non-disjunction, usually during gametogenesis. The error may occur, however, during cleavage, producing cells with different chromosome complements (mosaicism). Visible structural abnormalities of chromosomes result from deletions or translocations of chromosome fragments.