Acid cysteine proteinase inhibitor (ACPI or cystatin A) is a protein (12 kDa) which inhibits the action of several cysteine proteinases, e.g. cathepsins B, H, L and S. In this study the cellular location of ACPI has been immunohistochemically investigated in the normal human prostate, in benign prostatic hyperplasia (BPH) and in adenocarcinoma. ACPI was found in the basal epithelial cells of the normal prostate. The secretory epithelial cells did not express ACPI. In the hyperplastic prostate, the expression of ACPI was decreased and it was also expressed more focally in the basal cells. Hyperplastic basal cells also expressed ACPI. In prostatic adenocarcinoma, no ACPI expression was found. The absence of ACPI expression was obvious and if the sections contained both benign and malignant cells, only the benign glandular structures always expressed ACPI. The results suggest that expression of ACPI might be related to prostatic epithelial cell proliferation and differentiation. Possibly the detection of ACPI in tissue sections might be helpful in identifying prostatic adenocarcinoma, especially in cases with small carcinomatous foci.