Abstract The associations of denial caries with poor oral hygiene and high sugar consumption were analyzed taking into account possible confounding and factor interaction. The series consisted of 543 children from low-fluoride areas (0.10–0.46 parts/106), aged 5, 9 and 13 years Plaque accumulation and sugar consumption were slightly confounded throughout the observations. Effect-modification appeared to exist, since the effect of one factor was greater at higher levels of the other. The association between the amount of plaque and dental caries was statistically significant at all levels of sugar consumption. With increasing total sugar consumption the risk of caries increased significantly only when oral hygiene was simultaneously poor. Effect estimates (E) and attributable risk estimates (AR) were calculated for increased plaque accumulation and sugar consumption. For the total sets of tooth surfaces in the various age groups the proportions of the total caries load associated with increased plaque accumulation were 35.2–63.0%, and those associated with higher total sugar consumption 0.7–5.4%. The fractions varied greatly with the tooth group. The effect estimates for the two factors in combination were always greater than the sums of the separate effects, indicating synergistic interaction between the two caries determinants.