The objective of this study was to investigate the expression of macrophage migration inhibitory factor (MIF) in patients with colorectal cancer (GIS) and malignant melanoma (MM). The study group consists of pathologically verified colorectal cancer (n = 63) and malignant melanoma (n = 65) patients and healthy controls (n = 25). Serum MIF concentrations were determined by enzyme-linked immunosorbent assay. Serum values of the patients were significantly higher than the controls (p < 0.001 for GIS, p = 0.032 for MM). Diagnostic sensitivity and specificity were calculated for MIF for colorectal and malignant melanoma. The results demonstrate that colorectal cancer express and secrete large amounts of MIF.