Six patients, five with acute myeloid leukemia (AML) and one with a myelodysplastic syndrome (MDS), were found to have monosomy 7 by conventional cytogenetics at diagnosis. Repetitive DNA sequences from the heterochromatic region of human chromosomes 1 and 7 were used as probes for in situ hybridization experiments on interphase cells of these patients. A double hybridization protocol was used to reveal the particular chromosomes as distinct spots or clusters of signals within interphase nuclei. The chromosome 1 sequence served as an internal control. Simultaneous detection of the sequences showed the presence of two normal number 1 chromosomes and a missing 7 chromosome from individual cells. While cytogenetic preparations showed only -7 metaphases in 3 AML and 1 MDS patients, in situ hybridization of interphase cells showed many normal cells as well as the presence of -7 in fully mature granulocytes. One AML patient studied in remission showed only normal metaphases yet had 9% interphase cells with a missing 7 and relapsed within 3 months. We conclude that examination of interphase cells by in situ hybridization provides clinically useful data since every cell including mature granulocytes can be examined, the lineage of a cell can be determined, and efficacy of differentiation therapy can be evaluated.