This study aimed to (1) record the plaque and gingival status on sound and carious approximal surfaces and (2) investigate whether the two parameters could predict approximal lesion progression. For this purpose two samples were selected among 18- to 25-year-old patients at the Dental Faculty in Copenhagen. Sample 1 consisted of 45 patients, each having an approximal posterior surface requiring operative treatment and a contralateral approximal space, where both surfaces were radiographically sound. Sample 2 consisted of 25 patients, each having a posterior approximal tooth surface with an enamel/initial dentinal lesion recorded on a bitewing. Plaque and gingival status were classified using 4-point ranked scales. The recordings were performed once on each participant in sample 1. The recordings in sample 2 were repeated 5 times during a 15-month period. In order to assess lesion progression in sample 2, two serial intraoral examinations were performed, i.e. at the beginning and end of the study. Less than 10% of the sound and carious surfaces were recorded as plaque-free, while about 40% had a thick or heavy plaque accumulation. No significant difference was observed between plaque scores on sound and carious surfaces (p = 0.5), in contrast to findings representing the gingival status (p < 0.001). Tooth surfaces requiring operative treatment were associated with bleeding of the gingiva after probing. In sample 2, 3 of the 25 participants did not finish the study. On the remaining participants caries progression was observed in 9 cases (41%), while no progression was observed in 13 cases. Analyses showed that there were no significant differences between plaque scores in cases with caries progression, as compared with no caries progression (p > 0.05). Similar analyses concerning gingival status showed that bleeding of the gingiva was related to cases with progressing caries. Analyses also disclosed no direct association between plaque and gingival status (Spearman correlation coefficient 0.39). Thus, the occurrence of plaque does not have the same predictive power as bleeding after probing in decision-making as to whether or not a lesion progresses.