The purpose of this in vitro study was to test the accuracy and the reproducibility of diagnostic and treatment decisions of fissure caries with and without explorer. 34 dentists were asked to diagnose 61 teeth and decide upon possible treatment. The teeth were then histologically prepared and diagnosed. The agreement between histological and clinical diagnoses was assessed. The results showed no difference in diagnostic accuracy between explorer and visual technique only. Sensitivity (62%) and specificity (84%) showed that the dentists were more likely not to treat decayed teeth than to restore sound teeth. The percentage ‘correctly diagnosed teeth’ was approximately 42%. As there was an inherent possibility of a correct diagnosis by chance, this value had to be corrected to 23% (kappa statistics). The percentage of ‘clinically’ correct treatment decisions, however, was 73%. The reproducibility test gave kappa values of 0.47 for diagnostic and 0.44 for treatment decisions. It was concluded that the use of an explorer does not improve the validity of the diagnosis of fissure caries when compared to that of a visual inspection alone.