Inhibin is a peptide hormone which is produced by ovarian granulosa cells during normal follicular development. It is important that granulosa cells are recognized in fine needle aspirates (FNAs) of ovarian cystic lesions, as this allows definite recognition of a functional cyst and exclusion of a potentially neoplastic epithelial lined cyst. Occasionally the distinction between granulosa and epithelial cells may be difficult, especially when aspirates from functional cysts are unusually cellular. In the present study, FNAs from 33 ovarian cystic lesions were immunostained with a monoclonal antibody against inhibin. Nine cases of peritoneal fluid containing malignant cells in patients subsequently confirmed to have ovarian adenocarcinoma were also stained. Where possible the cytological and immunocytochemical findings were correlated with subsequent biopsy. In most cases in which cytology suggested a functional cyst there was a strong positive staining with anti-inhibin, although occasional cases were negative. One case originally thought to contain epithelial cells stained strongly positive with anti-inhibin and on review was felt to represent a cellular functional cyst. In all other cases where cells were considered to be epithelial there was no staining with anti-inhibin. The study shows that immunocytochemical staining with anti-inhibin may be of value in confirming the presence of granulosa cells, thus establishing a diagnosis of functional cyst. Although negative staining does not exclude a functional cyst, positive staining with anti-inhibin allows exclusion of an epithelial lined cyst and may avoid unnecessary surgical intervention.