Pathologists frequently need to judge whether basal cell carcinomas have been excised adequately. Traditionally, excision adequacy is assessed by looking for the presence of tumor at the margins of resection. This time-honored activity has questionable value, since it has been demonstrated that the majority of tumors with positive margins do not recur, and a substantial minority of tumors with negative margins do recur. It is proposed that excision adequacy can be evaluated by considering the pattern of tumor growth. Tumors composed of widely dispersed nests need wider margins than tumors that grow as tight clusters of tumor nests, and this assertion can be evaluated statistically. A morphometric study of 28 basal cell carcinomas (BCCs) was performed, in which the distribution of tumor cell nests seen in cross-section was analyzed. The average distance from the center varied greatly (272 to 2273 microns) among these tumors. Standard deviations were calculated from distances between the tumor center to each nest within the tumor, and one-tailed Student's t tests were used to obtain 90%, 95%, and 99% confidence limits for distances beyond which no additional tumor nests are expected. These distances, in tumor radii, ranged from 0.8 to 1.9 and from 1.03 to 4.89, for 90% and 99% confidence limits, respectively. Conventional methods used to determine margin adequacy do not account for the discontinuous appearance of BCC in histologic sections. This theoretical model demonstrates that an alternate way of assessing excision adequacy can be achieved with a statistical analysis of the pattern of tumor growth, rather than looking for absence of tumor at the resection margin.(ABSTRACT TRUNCATED AT 250 WORDS)