Examination of routinely stained haematoxylin and eosin sections may sometimes prove inadequate in differentiating partial hydatidiform moles (PHM) from complete hydatidiform moles (CHM). While cytogenetic analysis can aid in the distinction, such facilities are not always available. The possibility of using immunohistochemistry to aid in the differentiation was studied. Twenty-five histologically proven CHM and 11 PHM were studied for their patterns of expression of human chorionic gonadotrophin (hCG), human placental lactogen (hPL) and placental alkaline phosphatase (PIAP). All CHM stained diffusely with hCG and focally with both hPL and PIAP irrespective of gestational age. Of PHM, 63.6% were diffusely positive for hCG, 27.3% for hPL and 54.5% for PIAP; the rest were focally positive. The hCG pattern changed from diffuse to focal with increasing gestational age of PHM, while those of hPL and PIAP became increasingly diffuse with gestational age. While these protein expressions may be applied in differentiating late PHM from CHM, it is not useful in first trimester cases. The most helpful application is that focal expression of hCG and diffuse expressions of hPL and PIAP is not seen in CHM, thereby excluding such a diagnosis. PHM, in contrast, can show either diffuse or focal expression of all 3 antigens.