We have studied the value of additional immune suppression in BMT conditioning regimens in 45 patients with leukemia and 4 with myelodysplastic syndrome allografted between 1984 and 1991. A dose of 6 Gy total lymphoid irradiation (TLI) was delivered to 12 of 24 and 15 of 25 patients conditioned with 10 Gy and 12 Gy total body irradiation (TBI), respectively. Thirteen patients also received methylprednisolone (MP) before BMT to enhance immunosuppression. Differences in immunosuppression between the TBI with or without TLI or MP regimens and the influence of different levels of graft T cell depletion were measured in terms of transplant rejection, and complete versus mixed chimerism. The treatment-related complications were evaluated in terms of GVHD and incidence of pneumonitis. The overall transplant rejection rate was 6% (3 of 49). Complete chimerism was not significantly modified by increasing the TBI dose or by additional TLI (p > 0.10) but was more often seen in patients receiving MP given as pre-transplant immunosuppressor booster (p = 0.01). The incidence of GVHD was only influenced by the level of T cell depletion (p = 0.003). All 49 patients received a TBI lung dose in the range 9.5-10 Gy. A crude pneumonitis range of 19% (9 of 47 evaluable patients) was found. Neither the addition of TLI, MP nor the T cell depletion influenced the lung toxicity rate (p > 0.10) but pneumonitis was more frequent in patients with GVHD (p = 0.005).