The length of the human Y chromosome was measured and compared with the mean length of the group E (16–18) chromosomes in ten cells from each individual in the following groups of subjects: 30 control individuals; nine members of three randomly chosen families; ten members of four families with extremely long Y chromosomes; and ten members of three families with extremely long Y chromosomes and a G-trisomic index patient. Measurements were performed a second time on three individuals from the control sample. The results obtained were treated statistically and the following conclusions could be drawn: (1) Analysis of variance indicated good reliability of the method. The mean value of the standard deviations for the variation of the duplicated measurements was 0.027. (2) Although there was a slight intercell variation within the individual, the coefficient of variation was below 5% in all 59 individuals involved in the study. Thus, the length of the Y chromosome was constant within the individual. (3) In the control population there were highly significant interindividual differences, the mean values ranging from 0.55 to 0.88 (Y/E ratio). Although a mean value for the population can be calculated (0.7306 ± 0.012), this mean has little significance because of the wide variability in the length of the Y chromosome. The distribution is of a Gaussian type without step formation. (4) No family showed any statistically significant interindividual variation. It thus could be concluded that the Y chromosome is inherited at a constant length. This applies to randomly chosen families, families with extremely long Y chromosomes and families with long Y chromosomes in which there is an index patient with G-trisomy as well. That such a wide variation in the length of the human Y chromosome is possible can be ascribed to its heterochromatic condition, its presumed paucity of genes and that the greater part of it remains unpaired at meios