Macrophage migration inhibitory factor (MIF) has been shown to play an important role in the growth and metastasis of prostate cancer. The objective of this study was to determine whether the serum level of MIF could be used as a diagnostic biomarker for prostate cancer as well as a predictor of disease progression. A total of 369 men who underwent systematic prostate biopsy from January 2000 to April 2004 and 30 healthy controls were included in this study. The serum MIF level was measured using an enzyme linked immunosorbent assay. Associations between the serum MIF level and several clinicopathological factors were analyzed. Among 359 patients, 137 were diagnosed as having prostate cancer. The mean values of serum MIF in the control, benign and cancer-groups were 2.1, 3.5 and 10.8 ng/ml, respectively. The MIF levels of patients with prostate cancer were significantly higher than those of the other two groups. A comparison of MIF and PSA values in all patients showed a positive correlation (correlation coefficient r(2)=0.56, p<0.0001). The MIF value in patients with prostate cancer was significantly associated with clinical T stage, Gleason score and percentage of positive biopsy core (PPBC). MIF levels in patients with metastasis were significantly elevated compared with those in patients without metastasis. Among patients undergoing radical prostatectomy, the level of MIF in those with pathologically confirmed extraprostatic disease was significantly higher than that in patients with organ-confined disease. Moreover, multivariate analysis showed that MIF values, PSA values and PPBC were independent predictors for extraprostatic disease. These findings suggest that an increased serum MIF level is closely associated with the progression of prostate cancer, and thus the serum MIF level could be useful as a novel biomarker for the detection of prostate cancer as well as a predictor of disease progression.