Radiographs were taken of 580 unfilled approximal surfaces of all types of extracted permanent teeth with small carious lesions. The reproducibility of radiographic scoring was studied. More lesions were detected by inspection than on radiographs. Cavities were found in 34% of surfaces with radiolucencies limited to the outer enamel and in 66% when the radiolucent area had reached the inner half of the enamel. In surfaces with radiolucent areas involving the outer half of the dentine, cavities were found in 87% of the cases. The inciso-apical extent of lesions and the intensity of brown discoloration increased with increasing depth of lesion, but no such relationship was found for the bucco-lingual extent which seemed to depend on the bucco-lingual width of the teeth. Sections (3 mm wide), prepared from 24 surfaces with small lesions, revealed only 10 small lesions imperceptible on the radiograph before sectioning. The findings are compared with the results of other investigations.