As far as the authors are aware, no large-scale clinical studies of free smooth surface caries have been carried out in the past, although carious lesions on these surfaces can be diagnosed at an 'early' stage. In this study on a group of 2854 13-year-old children, caries affecting the free smooth surfaces (buccal and lingual) was found to be a common problem, with a total of 2917 lesions observed on molar teeth at the first examination. Most of the lesions involved no macroscopic loss of tooth contour. When the children were examined 2 years later, three-quarters of the lesions had either remained static or regressed. Free smooth surface lesions were more likely to develop initially, and more likely to progress, in individuals with poor oral hygiene. Free smooth surface caries experience showed a positive correlation with occlusal and approximal decay experience. The number of free smooth surface carious lesions can be used as a predictor of caries risk. Strategies for diagnosis and prevention are suggested for patients presenting with free smooth surface caries on molar teeth.