A morphometric study of cytologic preparations from patients with benign and malignant (mesothelioma and carcinoma) pleural effusions is reported. The routine cytologic smears from these specimens were studied with a new system of video-based computerized interactive morphometry (CIM) that allows the measurements of real-time images of cell profiles by the simple procedure of touching the two extreme points of a diameter of interest on a touch-sensitive screen. For each cell, the nuclear profile diameter (NPD) and the cytoplasmic profile diameter (CPD) are measured and categorized into classes with 2-microns intervals; the NPD/CPD ratio is also calculated. The mean NPD is calculated for the specimen after measurement of 100 cells. The data were interpreted by two independent methods: a statistical method of discriminant analysis that classifies the lesions as benign, carcinoma or mesothelioma and provides a probability statement of membership in a particular diagnostic class and an ad-hoc algorithm that categorizes the effusions as benign or malignant based on hierarchic analysis. A data base derived from study of the first 24 cases was constructed and utilized for the test classification of the second 24 cases, which were treated as specimens of unknown diagnosis. The discriminant analysis correctly classified 21 of the 24 test cases into their proper diagnostic groups. The algorithm for a computer-generated pathologic diagnosis correctly identified 47 of the 48 cases as benign or malignant. The technical advantages of video-based CIM over the existing morphometric methods are discussed.