The distribution of caries among homologous surfaces can exhibit three possible patterns: random, aggregated, or regular. In a random caries pattern, caries lesions are randomly distributed among homologous surfaces. An aggregated caries pattern is distinguished by the aggregation of lesions on one side of the mouth or the other to a greater extent than would be expected by chance alone. For a regular caries pattern, the left-right distribution of lesions is more homogenous than would be expected by chance alone. A test statistic based on the left-right distribution of caries lesions among discordant homologous pairs was developed to investigate which of these three caries patterns is present in a representative sample of the adult United States population. The data originated from the National Survey of Oral Health in the US (Employed Adults), 1985-1986. Of the 15,132 subjects studied, 12,776 subjects had 2 or more decayed or filled teeth. (At least 2 carious or filled teeth are required for detection of patterns.) Approximately 50% of these subjects (n = 6,439) had two or more discordant homologous tooth pairs. With these tooth pairs, the hypothesis of a random caries pattern was rejected in favor of an aggregated caries pattern (p < 0.0001). Similar findings were obtained with discordant homologous surface pairs. This aggregation of caries on one side of the mouth or the other may be due to genetic, infectious, and/or environmental factors.