Cellular DNA in skin biopsies from eighteen patients with positive patch tests and eleven patients with benign lymphocytic skin infiltates (cutaneous lymphocytoma, lymphoplasia or Jessner's lymphotic infiltrate) was estimated by flow cytometry of propidium iodide-stained dermal cells. Specimens from three patients with patch tests and one with cutaneous lymphocytoma had DNA histograms similar to those of normal blood mononuclear cells. The remaining patients demonstrated DNA histogram abnormalities in the form of hyperdiploid peaks in forty-two of forty-five specimens. The estimated proportion of cells in the S-phase was similar in patch tests (median: 9.2%) and in benign lymphocytic skin infiltrates (median: 9.6%). The median number of cells in the G2/M-phase was 3.1% in both groups of disorders. These data indicate DNA content heterogeneity of the dermal cells in benign lymphocytic skin infiltrates. This may be attributed to DNA replication and/or chromatin dispersion during cellular activation and proliferation. The data also emphasize that hyperdiploid peaks are not necessarily indicative of the presence of separate cell clones.