Thick cell clusters are a common finding in reactive and malignant effusions. In order to arrive at a diagnosis, clusters are evaluated for certain cytomorphologic features including size, shape, smooth vs. scalloped borders, and three-dimensional (3-D) configuration. By conventional microscopy, the image of these clusters is often blurred due to limitations in resolution. Consequently, the exact internal structure and cellular arrangement within these clusters cannot be adequately determined. Utilizing confocal laser scanning microscopy (CLSM), we examined serous fluids from a variety of conditions. Cases included mesothelioma, adenocarcinoma, and papillary adenocarcinoma. Smears were stained with 0.01% ethidium bromide and 1% eosin Y, followed by analysis with an ACAS 570 image analyzer (Meridian Instruments, Inc. Okemos, MI). Serial confocal fluorescence images were acquired, which allowed 3-D reconstruction of the clusters. Mesothelioma clusters (excluding those with obvious central collagen cores by light microscopy) appeared to be formed of the following configurations: 1) randomly coiled cords of cells, 2) small papillae encompassing central cores, and 3) tissue fragments with pseudoacinar formation. In contrast, adenocarcinomas had a more orderly pattern, with tightly cohesive cells and true acinar formation.