Expression of transforming growth factor-β (TGF-β), which inhibits the proliferation of hematopoietic progenitors, was investigated simultaneously with cell cycle characteristics in 63 bone marrow biopsies from 23 cases with acute promyelocytic leukemia (APL). Bromodeoxyuridine (BrdU) was administered to every patient (17 newly diagnosed) for determination of the labeling index (LI) and the durations of S-phase (Ts) and the cell cycle (Tc) of leukemic promyelocytes. APL cases had lower LI both in the bone marrow aspirate (6.1% v 11.4%, P = .008) and biopsy (21.1% v 28.0%, P = .001) and longer Tc (93.6 hours v 56.0 hours, P = .002) when compared with other French-American- British subtypes. TGF-β expression (detected by a monoclonal anti-TGF- β2/β3 antibody) was dramatically high, especially in interstitial areas of the biopsies. S-phase cells were found as geographically restricted islands of proliferation (GRIPs) in 20 of 22 cases. Weekly biopsies showed an increment in TGF-β on day 7 of therapy in 13 of 17 cases, while in vivo differentiation was noted in 9 of 15. We conclude that the presence of high TGF-β expression may explain the biologic basis for the slowly cycling nature of leukemic promyelocytes in APL as well as the unique clustering of S-phase cells observed in GRIPs.