The clinical significance of low numbers of aneuploid cells in routine cytogenetic studies of cultured lymphocytes is not always clear. We compared the frequencies of chromosome loss and gain among five groups of subjects whose karyotypes were otherwise normal; these groups were (1) subjects studied because of multiple miscarriages,(2) parents of live borns with autosomal trisomy,(3) subjects studied because they had a relative with Down syndrome,(4) an age-matched control group of phenotypically normal adults studied for other reasons (eg, parent of a dysmorphic child or member of a translocation family), and (5) other mostly younger and phenotypically abnormal subjects who could not be assigned to the first four groups (eg, individuals with multiple congenital anomalies or mental retardation). No significant age, sex, or group effects were observed for autosomal loss (hypodiploidy) or gain (hyperdiploidy). Autosomal loss was inversely correlated with relative chromosome length, but autosomal gain was not. Sex-chromosome gain was significantly more frequent in females than in males, but sex-chromosome loss was not significantly different between the sexes. Significant age effects were observed for both gain and loss of sex chromosomes. When age and sex were accounted for, the frequencies of sex-chromosome loss and gain were not significantly different among the five clinical groups. In general, low numbers of aneuploid cells are not clinically important when observed in blood chromosome preparations of subjects studied because of multiple miscarriages or a family history of autosomal trisomy.