Three patients with male habitus, short stature and testicular differentiation are described. All had mos 45,X/46,XY, the ratio of the two stemlines varying between the patients and between different tissues. The Y chromosome was abnormal, lacking the brilliant QFQ fluorescence and dark CGB staining characteristic of the distal part of the normal Y. Detailed banding studies suggested that the short arm and proximal part of the long arm were normal, while the distal part of the long arm was molecularly or otherwise altered, resulting in abnormal staining properties. Two of the patients were tested for H-Y antigen and found to be positive. These data and those collected from the literature are compatible with a model in which the primary lesion in X/XY mosaicism is a molecular alteration in the reiterated Y-specific DNA sequences (and possibly neighbouring sequences) of a 46,XY zygote resulting in the frequent mitotic loss of the Y and the emergence of a 45,X line. Provided the testis-determining gene(s) near the centromere are normal, testes are formed and the patient is H-Y antigen-positive. The extent of male or female differentiation depends in part on the prevalence, time of occurence, and distribution of the 45,X line and possibly in part on the alteration of other genes involved in sex differentiation and located on Yq further from the centromere.