This study compared conventional light microscopy with immunohistochemistry in the histopathologic diagnosis of intrauterine pregnancy in curettings in which fetal parts and chorionic villi were absent. Hematoxylin and eosin-stained sections of the curettings, which were from 50 consecutive patients in whom incomplete abortion had been diagnosed clinically, were circulated to four pathologists who graded their diagnoses with a confidence score. Immunohistochemical examination using a standard streptavidin-biotin-peroxidase method with anti-HPL and antikeratin antisera was performed. The pathologists in the maternity hospitals achieved a high level of diagnostic confidence compared with those working in the general hospitals. However, there were erroneous diagnoses by the one pathologist in the former group and none by the latter. Critical path analysis showed that the best performing pathologist could accurately diagnose all but two of the cases that had been diagnosed with a degree of doubt by the other pathologists without recourse to immunohistochemical examination. These results suggest that immunohistochemistry may be used discriminately in uncertain cases or if relatively inexperienced pathologists are reporting.