Total lymphoid irradiation (TLI) has been shown experimentally to induce a state of partial tolerance when administered before organ transplantation. Anecdotal reports in clinical transplantation have suggested efficacy of TLI in the treatment of recurrent rejection after heart transplantation. To further assess the safety and efficacy of TLI, 19 patients were entered into a protocol of TLI for the treatment of recurrent or early severe rejection despite conventional therapy. Rejection rate decreased from 1.3 episodes/month before TLI to 0.53 during TLI and 0.07 after TLI (p < 0.0001). Infections increased during TLI (possibly related to recent augmented immunosuppression before TLI), but all infections were successfully treated. One death occurred after TLI from acute allograft rejection. White blood cell (WBC) and platelet counts were depressed during and after (3 months) TLI. Frequent adjustments of dosing interval and, occasionally of the dosage were required to control WBC and platelet counts. Five patients experienced transient WBC of less than 1000/ml. More rejection episodes (and thus greater overall immunosuppression) before TLI and a lower tolerated dose of azathioprine before TLI predicted (by multivariate analysis) a lower WBC during TLI. Conclusions: (1) TLI is an effective adjunct for the intermediate control of early or recurring acute allograft rejection. (2) Close surveillance of WBC and platelets with appropriate adjustment of TLI dose and interval is necessary during TLI therapy. (3) The long-term benefits, possible late deleterious effects, and potential role of LTI as induction therapy remain to be elucidated.