The chromosomes were studied in four patients with Marfan’s syndrome, three male and one female, and, for comparison, in ten normal males. The karyotypes of the patients with Marfan’s syndrome were normal in their general features, but systematic analysis of the lengths of chromosomes 1–2, the Y, and 21–22 revealed that the chromosomes 21–22 of the patients were relatively shorter than those of normal persons (Table I). The Y of the three male patients with Marfan’s syndrome also varied, in one it was longer (Fig. 1), and in another shorter, than in the controls. The statistical significance of these findings was tested (Tables II–VII). It was concluded that, at least in the cases of chromosomes 21–22, genotypic control of chromosome length, rather than structural variation, was underlying the differences observed between the patients with Marfan’s syndrome and the normal controls.