The objective of this study was to evaluate the impact of cyclosporine (cyclosporin A; CyA) prophylaxis for graft-versus-host disease (GVHD) on the development of obstructive airways disease (OAD) after allogeneic bone marrow transplantation (BMT) in leukemic patients. Patients with normal pulmonary function tests (PFTs) prior to BMT were followed with serial PFTs for the development of OAD. Follow-up PFTs were performed at 3, 6, 9, and 12 months, and thereafter at consecutive yearly intervals. Obstructive airways disease was defined as FEV1 less than 80 percent, ratio of FEV1 over the forced vital capacity (FEV1/FVC) less than 80 percent of predicted, maximal midexpiratory flow rate at 50 percent vital capacity less than 65 percent of predicted, or residual volume greater than 120 percent of predicted. In the period prior to CyA prophylaxis for GVHD development (March 1983 to September 1986), 17 (39 percent) of the 44 patients undergoing BMT developed OAD, compared with 2 (4 percent) of 45 in the post-CyA period (September 1986 to March 1990) (chi 2 = 17; p < 0.00005). Age, sex, type of leukemia, severity of GVHD, histocompatibility status, presence of acute GVHD, and sex mismatch between donor and recipient were not associated with development of OAD. Although chronic GVHD was associated with OAD in univariate analysis, a multivariate logistic regression analysis showed that the only significant independent predictor for OAD was the use of CyA. We conclude that CyA is protective against the development of OAD after BMT in leukemic patients.