Objectives: Dental caries is among the most common chronic diseases of the childhood. This study sought to assess the effect of caries experience in primary molars on caries development in the adjacent permanent first molars. Materials and Methods: This cross-sectional study evaluated 413 students aged 7 and 8 years. Clinical dental examination was performed by two independent examiners using disposable dental instruments and a head light. Dental caries was evaluated using the International Caries Detection and Assessment System (ICDAS) for all four permanent first molars and the decayed, missing, and filled teeth (dmft) index for the primary molars. Data were analyzed using the Pearson's Chi-squared test and Fisher's exact test. Results: When primary first molars were sound, 22.9% of the permanent first molars of the same quadrant were sound. When primary second molars were sound, 25.7% of the adjacent permanent first molars were sound. A carious primary second molar had a stronger correlation with development of dentin caries in the adjacent permanent first molar than a carious primary first molar (P<0.001). Conclusion: The current results confirmed that carious primary molars can significantly affect caries development in the adjacent permanent first molars, and a carious primary second molar has a significantly greater effect than a carious primary first molar in this respect. Thus, special attention should be paid to oral hygiene and proper tooth brushing of primary molars, particularly second molars.