The authors review the natural occurrence of tumour dormancy in man and animal models, which show the importance of cellular defence mechanisms in inducing and maintaining the state of tumour dormancy. Six cases of non-small-cell lung cancer are described. The patients exhibited moderate to very strong delayed hypersensitivity reactions to soluble lung-cancer antigen after specific immunotherapy. Three patients had nonregional metastases at 11, 12 and 14 years. Two had regional recurrence after 9 years, and in one patient a small hilar-node metastasis was found at autopsy after 7.6 years. In each case an immunodepressive event or drug treatment preceded resurgent growth. The effect of renal transplantation in patients with a history of surgery for cancer supports the conclusion that cellular defence mechanisms are crucial for the maintenance of tumour dormancy in man.