The potential beneficial effects of partial lung shielding were investigated in 44 patients conditioned by whole body radiotherapy as part of the conditioning for bone marrow transplantation (BMT). The first 23 patients were irradiated with fractionated total body irradiation (TBI) to a total dose of 12 Gy. In the second series of 21 patients a 50% transmission lung shield was utilized following 6 Gy TBI. This treatment was combined in the second group with four fractions of 1.5 Gy total lymphoid irradiation (TLI), given in 2 d. The rate of idiopathic interstitial pneumonitis (IP) was 26% in the first group, compared to none in the second.