A study was designed to evaluate the usefulness of computer-aided image analysis in distinguishing benign from malignant pleural effusions. Morphometric measurements of preparations stained by the routine Leishman technique and quantitation of nucleolar organizer regions (AgNORs) were done on 65 cytologic samples randomly selected for the study and divided as follows: 30 samples from patients having benign pleural cytology and 35 samples from patients with pleural malignancies. The following parameters were obtained: nuclear/cytoplasmic ratio (N/C); nuclear and cytoplasmic area (NA and CA, respectively) and perimeter (NP and CP, respectively); major and minor nuclear diameter (DN and dN, respectively); major and minor cytoplasmic diameter (DC and dC, respectively); and absolute and fractional AgNOR (AgNOR/NA) area. Conformational characteristics of both nucleus (FFN) and cytoplasm (FFCy) were also computed. Statistical studies were done by means of discriminant analysis. The discriminant function generated from the morphometric data correctly diagnosed 100% of the cases. The validity of the classification provided by the function was tested by a jack knife ("one out" procedure), which again classified all the cases correctly. Our data support the concept that quantitative morphologic studies could be used in the management of patients with pleural effusions.