Tunnel restorations have been advocated as a conservative treatment modality for early carious lesions. However, the question arises as to quantity of dentin that remains within the marginal ridge above the tunnel path. In this experiment, extracted maxillary premolars with early carious lesions were sectioned and photographed. Measurements taken from tracings of the projected slides revealed that no dentin remained within the marginal ridge, irrespective of the direction of the tunnel path. These measurements indicate that the tunnel restoration is not a satisfactory treatment for proximal caries. The position of caries percludes the cutting of a tunnel preparation that leaves sufficient dentin under the marginal ridge, unless the bulk of enamel caries is left untouched.