Loss-of-function mutations in the gene coding for perforin (PRF1) markedly reduce the ability of cytotoxic T lymphocytes and natural killer cells to kill target cells, causing immunosuppression and impairing immune regulation. In humans, nearly half of the cases of type 2 familial hemophagocytic lymphohistiocytosis are due to bi-allelic PRF1 mutations. The partial inactivation of PRF1 due to mutations that promote protein misfolding or the common hypomorphic allele coding for the A91V substitution have been associated with lymphoid malignancies in childhood and adolescence. To investigate whether PRF1 mutations also predispose adults to cancer, we genotyped 566 individuals diagnosed with melanoma (101), lymphoma (65), colorectal carcinoma (30) or ovarian cancer (370). The frequency of PRF1 genotypes was similar in all disease groups and 424 matched controls, indicating that the PRF1 status is not associated with an increased susceptibility to these malignancies. However, four out of 15 additional individuals diagnosed with melanoma and B-cell lymphoma during their lifetime expressed either PRF1(A91V) or the rare pathogenic PRF1(R28C) variant (p = 0.04), and developed melanoma relatively early in life. Both PRF1(A91V)- and PRF1(R28C)-expressing lymphocytes exhibited severely impaired but measurable cytotoxic function. Our results suggest that defects in human PRF1 predispose individuals to develop both melanoma and lymphoma. However, these findings require validation in larger patient cohorts.