Differentiation between renal oncocytomas and well-differentiated oxyphilic cell renal carcinomas from fine needle aspiration cytology smears may present considerable difficulties. To better define diagnostic criteria, we used high-resolution architectural characteristics of nucleus and cytoplasm based on mathematical morphologic concepts. Particulate structural elements were selected for their size and optical density by the so-called top-hat function. Spatial density, size distribution and a function of distance between particles were computed. We quantitated cellular samples from fine needle aspirates obtained from 10 renal oncocytomas and 5 well-differentiated oxyphilic cell renal carcinomas. In two of the former, cytologic diagnosis was doubtful. Subsequent removal of the tumor and its histologic examination established the final diagnosis in all cases. Stepwise discriminant analysis correctly classified all cases as oncocytomas or renal carcinomas, although the results need to be considered with caution due to the necessarily limited number of observations. Multivariate graphic representation by star glyphs clearly depicted the differences between oncocytoma and oxyphilic cell renal carcinomas and the abnormal characteristics of two oncocytoma cases misclassified initially.