The DNA replication patterns of eight cases of X isochromosomes, five idic(X) and three i(Xq), were studied. R-banded prometaphases and metaphases from lymphocyte cultures after synchronization with methotrexate and incorporation of 5-bromodeoxyuridine were analyzed. No significant differences in the frequency of metaphases with symmetric and asymmetric replication patterns between dicentric and monocentric isochromosomes were found. Furthermore the distribution of the frequencies of R-positive bands was similar and comparable to that of the normal late-replicating X. Our data suggest that the DNA replication pattern of Xq isochromosomes is not correlated with the mechanism of their origin.