AIM--To determine whether aP2 expression is a useful diagnostic marker in soft tissue tumour pathology. METHODS--A polyclonal antibody to aP2 was used to investigate the immunohistochemical expression of this protein in benign and malignant tumours of adipocytic differentiation and a wide variety of other neoplasms. RESULTS--aP2 was only expressed by lipoblasts (in all types of liposarcoma and in lipoblastomatosis) and by brown fat cells (in both hibernomas and normal periadrenal fetal fat). Other benign adipose tissue tumours and malignant connective tissue or epithelial tumours were distinguished from liposarcoma by the absence of staining for aP2. CONCLUSION--Identification of lipoblasts using markers of aP2 expression is of value in the differential diagnosis of benign and malignant adipose tissue tumours and in distinguishing liposarcomas from other malignant mesenchymal and epithelial neoplasms, some of which contain cells that morphologically resemble lipoblasts.