Clinical evidence of a graft-vs.-tumour effect in solid tumours after haematopoietic stem cell transplantation is lacking. We report for the first time a complete and durable regression of a stage IB non-small-cell lung carcinoma in a patient who had received an allogeneic peripheral blood haematopoietic stem cell transplant for acute myeloblastic leukaemia in first complete remission. Disappearance of the tumour coincided with development of graft-vs. -host disease. This suggests that simultaneous generation of cytotoxic T lymphocytes against lung carcinoma cells could have been responsible for the regression. This unique clinical observation broadens the possibility of using allogeneic haematopoietic stem cell transplantation in treating neoplasias lacking significant sensitivity to chemotherapy.