Background The mesial surface of the first permanent molar is the most caries‐susceptible proximal surface of the permanent dentition in children under the age of 12. Aim The aim of this study was to determine the association between caries progression on the mesial surface of the first permanent molar (T6M) and caries on the distal surface of the primary second molar (t5D) and the occlusal surface of the first permanent molar (T6O). Design Children (between 5 and 13 years old; N = 565) that had participated in a 4‐year longitudinal caries study that at baseline had at least one T6 fully erupted with a t5 in proximal contact, with no restoration or sealant on T6O and t5D, and adequate bitewing radiographs were included. Clinical data using the International Caries Detection and Assessment System (ICDAS) and radiographs were used to determine the caries status of T6M, T6O, and t5D. Results Baseline caries presence on t5D and T6O were highly significantly associated with follow‐up caries presence on T6M (P < .001). The adjusted odds ratios corresponding to t5D and T6O were 3.94 (95% CI: 1.78, 8.71) and 3.26 (95% CI: 1.46, 7.31), respectively. Conclusion These findings highlight the need for prevention and management of caries on T6O and t5D.