Summary. Prenatal karyotyping using stimulated fetal blood lymphocytes was undertaken in 170 pregnancies between 16 and 36 weeks gestation for the following reasons‐(1) mosaicism or marker chromo somes found in amniotic fluid culture; (2) a family history of X‐linked mental retardation with fragile Xq28; (3) fetal abnormalities detected ultrasonographically; (4) late booking or amniotic fluid culture failure in patients with advanced age or balanced translocations; and (5) twin pregnancies discordant for a chromosomal anomaly. Forty‐one karyotypic abnormalities were detected (24%). These were: 45,X (7 cases). trisomy 13 (5 cases), trisomy 18 (6 cases), trisomy 21 (4 cases), twin pregnancy where one twin had trisomy 21 (1 case), supernumerary marker chromosome (3 cases, one of which occurred in a twin pregnancy). triploidy (3 cases), X‐linked mental retardation with fragile site at Xq28 in males (6 cases), fetal erythroleukaemia (3 cases including 2 cases with Turner's), Fanconi's anaemia (1 case), unbalanced chromosome translocation 47,XY+der22,t(l1;22) mat (1 case), mos 46,XXI8p‐/46,XX.‐18,+i(l8q) (1 case), 46,XXde1(2q) (1 case), and 46,XYt(5;17) de novo (1 case). In fetuses at high risk of a chromosome aberration. a rapidly obtaincd karyotype is helpful and fetoscopy and fetal blood sampling are justified in the second or third trimester.

Four cases of C.G.L. in which banding of the Ph1 chromosome was performed were found to have variation from the usual 9/22 translocation pattern. All 4 cases showed a rearrangement involving at least 3 chromosomes, 2 of which were a9 and a22. One of these cases had in addition an XYY karyotype in the bone marrow.

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Chromosomal abnormalities in three cases of chronic granylocytic leukaemia are presented. The relative importance of 'specific' chromosomal abnormalities, additional to the Ph1 in the karyotypic evolution of chronic granylocytic leukaemia is discussed. A new abnormal metacentric chromosome is described.