Prostate-specific antigen (PSA) is the most commonly used tumour marker for prostate cancer, both in screening and in follow-up. However, there are many false positive increases in the presence of other prostate diseases and, currently, there is no consensus regarding sensitivity and specificity of the PSA test, nor what constitutes the upper limit of normality. We report a case of a 67-year-old patient with metastatic prostate cancer who, with increased level of alkaline phosphatase and normal PSA, showed clinical and radiological evidence of progression of the disease.