We report five cases of metastasizing basal cell carcinoma. The incidence of metastases in all patients with basal cell carcinomas who had surgery in the same period was 0.1%. There was no squamous differentiation in the histology of any of the primary lesions, but we found some squamous differentiation in a late recurrence in one patient and in the metastases in two patients. All other histologic features were identical in both the primary tumor and in the metastases. Perineural spread and blood vessel invasion were evident in the primary tumor and/or recurrence of three of the five patients and may be important pointers to possible recurrence and metastasis. Of 205 cases of metastasizing basal cell carcinomas mentioned in the literature, we accepted and evaluated 170 cases of tumors of the skin with histologic proof of metastasis. The male-to-female ratio was 2:1, median age at onset of primary tumor, 45 years, median interval between onset of tumor and first sign of metastasis, 9 years, median age at first sign of metastasis, 59 years, and median survival after first sign of metastasis, only 8 months. The site of the primary tumors is similar to that in nonmetastasizing basal cell carcinoma. Metastatic spread is most often to lymph nodes, lungs, and bones with lymphogenic and hematogenic spread equally frequent. Age or sex of the patient seemed to have no influence on survival or way of metastatic spread. As metastasis of basal cell carcinoma is extremely rare (1 in 1,000 to 35,000), many authors sought or postulated histologic "degeneration" into a squamous cell carcinoma, but no such unequivocal evidence has ever been found. Areas of squamous differentiation were reported in less than 15% of both primary and metastatic lesions of metastasizing basal cell carcinoma. The mode of metastatic spread and the characteristics of the histology make it rather unlikely that metastasis of a basal cell carcinoma is due to a change toward squamous cell carcinoma.