A new method is described which makes it possible using skin specimens to perform flow cytometric analysis of DNA content. DNA content analysis was performed on 28 skin specimens and 9 blood samples from 18 patients with mycosis fungoides and Sézary syndrome. The reproducibility was fair, with almost identical results in 6 cases (mycosis fungoides and Sézary syndrome) where two samples (skin specimens or blood samples) were taken 6 hours to 10 days apart. Hyperdiploidy was found in 7 of 11 skin specimens from patients with mycosis fungoides stage I with negative histology. In 13 skin specimens and 3 blood samples from patients with mycosis fungoides stages II and IV, abnormalities including hyperdiploidy, near-tetraploidy and near-hexaploidy were found in 8 of 13 skin specimens and in 2 of 3 blood samples. Four patients with Sézary syndrome were studied: 2 patients in remission showed normal DNA histograms (2 skin specimens, 3 blood samples) and 2 patients with active disease showed aneuploidy in the 2 skin specimens examined and in 1 of 3 blood samples. These studies demonstrate: 1) the importance of flow cytometry as a diagnostic tool for use on skin specimens in the early stages of mycosis fungoides where routine histology is non-diagnostic; 2) the diagnostic and prognostic aid of flow cytometry during the course of mycosis fungoides and Sézary syndrome in addition to the probability of measuring the effect of treatment.