MIF loss promotes skin tumorigenesis. 6–10 week old male MIF-/- or MIF+/+ C57Bl/6 mice (n = 20 per group) were treated with 200 μg of the carcinogen benzo[α]pyrene (B[α]P) in 100 μl acetone topically on their backs once per week for 20 weeks. Skin tumors started to appear after week 14, and increased in number during the course of B[α]P treatment. MIF-/- mice developed nearly twice as many tumors per mouse as MIF+/+ controls. Based on histological evaluation by a blinded pathologist, this primarily reflected an increase of non-invasive tumors. By contrast, the number of invasive tumors was similar between the genotypes. Likewise, we found no significant difference with respect to tumor size or vascularization. * = statistically significant with p < 0.01 in a Student's t-test.