Partial regression in cutaneous malignant melanoma has been reported by a number of observers, albeit not all, to be associated with a relatively poor prognosis; in contrast, a keratoacanthoma, which eventually regresses, does not metastasize. The Hammond effect could explain the possibly poor prognosis of the thin regressing melanoma. Hammond(W.G. Hammond et al., Cancer J., 8: 130-138, 1995) showed that the speed of biological progression to less differentiated phenotypes is directly related to the immunocompetences of the tumor hosts. If partial regression is a sign of an unusually strong immune reaction, then the melanoma that partially regresses might have a relatively poor prognosis because of the greater risk of biological progression among the surviving tumor clones. A Hammond effect is not associated with regression of a keratoacanthoma. I postulate that the growth of this tumor is accelerated, rather than restrained, by the immune reaction and that the ultimate regression of the tumor is the result, not of immune cytotoxicity, but of a rapid terminal differentiation (a reverse Hammond effect); alternatively, very rapid growth might lead to an exhaustion of growth potential before progression to clonal immortality could occur.