The results of a cytogenetic and molecular reinvestigation of a series of 52 patients with Turner's syndrome are reported. No evidence of Y chromosome material was found among the patients with a 45,X constitution but two patients were found to have a cell line with a r(Y) chromosome which was previously thought to be a r(X). The parental origin of the single X in the 45,X patients was maternal in 69% and paternal in 31%, a similar ratio to that seen among spontaneously aborted 45,X conceptuses. This suggests that X-chromosome imprinting is not responsible for the two grossly different phenotypes associated with a 45,X chromosome constitution. Approximately half of the structurally abnormal X chromosomes were maternal in origin and half paternal. This observation is consistent with either a meiotic or post-zygotic mitotic origin and at variance with the predominantly paternal origin reported for autosome structural abnormalities.