A variety of gene analyses were performed on lymphoid tumors from transplant patients who received cyclosporine A for immunosuppression. Epstein-Barr virus DNA was detected in the tumors, and the structure of circular episomal virus DNA was used as a measure of cell clonality. This analysis was correlated with clonality determined by study of immunoglobulin gene rearrangement. Some of the tumors had DNA rearrangements near the c-myc gene. Analysis suggested the pathogenesis of the tumors and indicated four categories of lymphoproliferation, three neoplastic and one reactive.