Methods and immunoassays for the determination of fatty acid synthase (FAS) expression in patients having or suspected of having a proliferative disorder, especially prostate cancer, are disclosed. The sensitive method and assay detect the level of expression of FAS in a biological sample using antibodies that are highly specific for FAS. The method and assay can be used to monitor the progression of cancer, and/or to predict the efficacy of certain treatments or the likelihood of recurrence of the cancer.