Cutaneous biopsy specimens obtained from bone marrow transplant (BMT) patients, most with graft-vs-host disease (GVHD), were analyzed for infiltration by helper, cytotoxic, and suppressor T lymphocytes and natural killer cells. Lesional skin from patients with early mild GVHD and drug reactions showed a CD4/CD8 ratio of 5.0 or more, but later biopsy specimens from patients with acute GVHD and the majority of sections from those with chronic GVHD showed a CD4/CD8 ratio of 0.8 to 3.0 due to increased numbers of presumably cytotoxic cells. Significant numbers of suppressor (CD11 +/- CD16b-) cells were found in only one patient with severe chronic GVHD. Natural killer cells were not found. Preliminary examination of lesional skin from seven autologous BMT patients showed a similar trend of decreased CD4/CD8 ratios in the three patients with a syndrome that resembled GVHD. Analysis of CD4/CD8 ratios in serial biopsy specimens from patients with GVHD may allow more accurate monitoring of the progression of cutaneous GVHD and may help to elucidate the mechanism of development of the GVHD-like reaction in autologous BMT patients.