There is a changing conception concerning what constitutes conservative treatment need This means that it is possible for dentists to decide on different levels of treatment for primary caries in similar circumstances. A study was carried out in which nine matched pairs of teachers at the dental schools of Groningen (the Netherlands) and Cardiff (Wales) were asked to diagnose duplicated bitewing radiographs for caries, and caries in need of restorative treatment. The different diagnostic measures used showed that although sensitivity and specificity of the two groups of teachers were similar, at the first viewing the general diagnostic accuracy of the Groningen teachers was significantly better (mean Cohen's kappa ‘dentine caries’: Groningen 0·70, s.d. 0·06; Cardiff 0·61, s.d. 0·08; P<0.05) when compared with a validated norm. The Cardiff teachers made significantly more overtreatment decisions (Groningen mean 22 per cent, s.d. 9 per cent; Cardiff mean 32 per cent, s.d. 7 per cent; P<0·05). At the second viewing the two groups appeared to adopt different decision-making strategies: sensitivity for indicating ‘dentine caries’ in need of restorative treatment tended to increase for Groningen and decrease for Cardiff teachers between viewings (Groningen mean 76 to mean 78, Cardiff mean 70 to mean 64). The results suggest that there are differences in treatment philosophy between both the groups and individual teachers. The Cardiff teachers seem to have adopted a more stringent cut-off point when deciding on restorative treatment need. Recommendations are made as a result of this study for ways in which the diagnostic decision-making behaviour of dentists could be studied in the future.