Endodontal reactions due to caries lesions may cumulate with those of restorative procedures and cause unexpected endodontal complications. It was therefore the aim of this study to characterize semiquantitatively the endodontal reactions by a new complex coding system of pulpal pathosis. 79 teeth without marginal periodontitis and scheduled for extraction were investigated clinically as well as radiographically: Caries superficialis (9 teeth, group 1); Caries media (23 teeth, group 2); Caries profunda (43 teeth, group 3); no carious lesions (4 teeth, control group). The teeth were serially sectioned, stained with haematoxilin-eosin and according to J.Hopkins (bacterial stain). The sections were histologically investigated according to modified BRD criteria (J Dent Res 75:1230, 1996) including the parameters: (i) Bacterial invasion, (ii) Regenerative parameters, (iii) Degenerative parameters. Bacterial invasion into dentin tubules and degenerative changes did not occur in all types of carious lesions but was most pronounced in teeth exhibiting deep caries (BRD coding for bacterial invasion: group1/group2/group3/control: 0,13/1,05/2,34/0; BRD coding for irregular irritation dentin: group1/group2/group3/control: 0,67/0,82/2,00/0,50), whereas necrosis was exclusively found in group 3 teeth (BRD coding: 0,84). It is concluded that BRD criteria proposed for long term biocompatibility testing of restorative materials and techniques are also useful in the characterization of the histopathology of caries progression. Concerning the clinical caries diagnosis superficial caries induces typical degenerative but localized pathological reactions. Only deep carious lesions may cause severe reactions showing signs up to the desintegration of pulp tissue. For restorative procedures of deep carious lesions a limited reactibilty of the endodont has to be taken into consideration.