To determine whether there might be immunophenotypic differences between nodular lymphocyte predominance Hodgkin's disease (NLPHD) and progressive transformation of germinal centers (PTGC) to aid in the differential diagnosis, we compared 16 cases of NLPHD with 13 cases of florid PTGC and 2 cases of focal PTGC. Paraffin-section immunohistochemistry was performed for CD20, CD45RA, CD45RO, CD3, CD43, CD57, EMA, CD30, and CD21. All PTGC cases showed well-circumscribed nodules of confluent sheets of CD20+ CD45RA+ small cells. T cells were scattered singly or in small groups. In 5 patients with florid PTGC, the T cells in some of the nodules formed rings around a few large transformed lymphocytes. In contrast, the nodules in all NLPHD cases showed an irregular, "broken-up" pattern with CD20 and CD45RA, and there were prominent T cell rosettes around the CD20+ large cells in all nodules. Rosettes of CD57+ cells and staining of large cells for EMA were seen in 3 and 2 cases of NLPHD, respectively, but not in PTGC. There were no differences between NLPHD and PTGC with respect to staining for CD30 or CD21. Three of the eight patients with florid PTGC and a few T cell rosettes had had persistent or recurrent lymphadenopathy; NLPHD developed in 1 of these patients 13 years later. We conclude that a combination of pan-B and pan-T antigens can be a useful adjunct to morphology in distinguishing NLPHD from PTGC. In approximately one-third of florid PTGC cases, T cell rosettes may be present, but they are notably fewer than those in NLPHD. Close follow-up of such patients may be appropriate.