Spontaneous regression of cancer is reported in virtually all types of human cancer, although the greatest number of cases are reported in patients with neuroblastoma, renal cell carcinoma, malignant melanoma and lymhomas/leukemias. Study of patients with these diseases has provided most of the data regarding mechanisms of spontaneous regression. Mechanisms proposed for spontaneous regression of human cancer include: immune mediation, tumor inhibition by growth factors and/or cytokines, induction of differentiation, hormonal mediation, elimination of a carcinogen, tumor necrosis and/or angiogenesis inhibition, psychologic factors, apoptosis and epigenetic mechanisms. Clinical observations and laboratory studies support these concepts to a variable extent. The induction of spontaneous regression may involve multiple mechanisms in some cases although the end result is likely to be either differentiation or cell death. Elucidation of the process of spontaneous regression offers the possibility of improved methods of treating and preventing cancer.