Purpose: The purposes of this study were to determine the presence of interproximal carious lesions in the primary molars and assess the risk to adjacent and contralateral surfaces when caries is detected in one quadrant. Methods: Clinical records of high caries-risk children, including bitewing radiographs, were assessed for interproximal caries and the extent of marginal ridge breakdown. The carious lesions were scored radiographically by two calibrated examiners with excellent inter- and intraexaminer reliability. Correlations for the presence of adjacent surface caries and caries in another quadrant were determined by employing the strength of the associations, as quantified using chi-square analysis. Results: A total of 1,290 pairs of primary molars (A and B, I and J, K and L, S and T) were included. Only eight percent of interproximal surfaces were caries-free. Approximately 70 percent of the adjacent surfaces (A and B, I and J, K and L, and S and T) exhibited carious lesions in the enamel when there was enamel caries in one quadrant. Furthermore, 90 percent of the adjacent surfaces exhibited caries in the dentin if one adjacent interproximal surface exhibited dentinal caries. Conclusions: In high caries-risk children, only a small proportion of surfaces remain caries-free, and the adjacent surface caries prevalence remains high at 70 percent to 80 percent. The presence of interproximal caries in one quadrant is a good indicator of caries in the other quadrants, as is the marginal ridge breakdown which indicates the proximity to the pulp.