Fourteen pairs of extracted contralateral premolars with small, artificial, standardized, approximal 'caries lesions' were placed in 14 plaster jaws. Fourteen dentists made a mesial tunnel preparation and a distal composite resin preparation on one tooth and vice versa on the contralateral tooth. Less tooth substance was removed in the tunnel preparations than in the class-II preparations, but this difference was not statistically significant when the resin class-II preparations were made without occlusal retention. Twenty-five per cent of the tunnel preparations had residual 'caries', as opposed to 7% in the class-II composite preparations. Tunnel preparations with larger occlusal openings had less residual 'caries'. The morphology of the class-II resin preparations varied considerably, indicating a lack of précise descriptions in the dental literature.