The morphologic and staining characteristics of the sex chromatin bodies and the DNA replication patterns were studied in a patient with a 45,X/46,X,idic(X)(p22.3::p22.3) karyotype and in a normal woman. The analysis showed a relatively high frequency of bipartite Barr bodies as well as some variation of the distance, staining intensity, and size relationship between their halves. Regarding the DNA replication studies, in 71% of the cells the abnormal X chromosome showed a synchronous pattern, and in the remaining 29%, in which a slight asynchrony was present, an almost equal proportion of early and late functional and nonfunctional centric halves was observed. Furthermore, the atypical chromosome had a quite similar replication pattern to the late replicating X chromosome of the normal woman, suggesting that its sequence of DNA sythesis was not altered.