Light microscopic cytologic diagnosis of pleural and peritoneal effusions--specifically, the differentiation between atypical or reactive mesothelial cells and malignant cells--remains a problem. During the past several years, various labor-intensive special techniques have been used to obtain a better understanding of the morphology of these cell types. The present study, performed in part to provide a basis for developing more advanced quantitative cytologic techniques, employed visual light microscopic cytometry using an ocular micrometer on 750 malignant and 750 atypical mesothelial cells at a magnification of 1,000X. Our results statistically confirmed the importance of diagnostic features used in routine cytologic evaluation. In addition to the previously reported differences between cellular, nuclear and nucleolar size in malignant cells and atypical mesothelial cells in effusions, the percentage of irregular nucleoli was shown to be an important differential feature, averaging 58.8% in malignant cells as compared with 6.6% in atypical mesothelial cells.