Various methods of diagnosing occlusal caries have been described, but only visual, clinical examination and radiographic examination are commonly used in dental practice. This laboratory study investigated the ability of 12 examiners to detect the presence or absence of occlusal caries in 48 extracted molar teeth using these two techniques. The presence or absence of caries was subsequently verified by sectioning the teeth. Diagnosis from visual examination was poor, only 48.7% of lesions in dentine being detected. Radiographic diagnosis was better with 62.2% of dentine lesions being found. However, this increase in sensitivity of the diagnosis was accompanied by a decrease in specificity with the number of false positive diagnoses increasing when the radiograph was used. Therefore, although the bite-wing radiograph may be regarded as a safety net for the diagnosis of occlusal caries, it must be interpreted with caution bearing in mind the possibility of false positive diagnoses.