OBJECTIVE To investigate how the size of the space between restoration and dentinal wall of the tooth affects the development of secondary caries lesions, especially wall lesions. METHODS Tooth-resin composite specimens were mounted on custom-made gap model stages and divided into 4 groups (n = 10): group 1 with a 30-μm gap throughout both enamel and dentin, group 2 with a 30-μm enamel gap and 530-μm dentinal gap, group 3 with 525-μm gaps in both enamel and dentin, and group 4 with 525-μm and 1,025-μm gaps in enamel and dentin, respectively. Specimens were gas sterilized and incubated in a cycling microbial caries model for 8 days and analyzed with confocal microscopy for lesion size at the enamel outer lesion (EOL), enamel wall lesion (EWL), dentin wall lesion A (DWL-A) next to the dentin-enamel junction (DEJ) and dentin wall lesion B (DWL-B) at 750 μm from the DEJ. RESULTS No difference in EOL or EWL size was found between the groups. DWL-A and DWL-B were larger in group 3 than groups 1 and 2. A larger DWL-B was found in group 3 than group 4. CONCLUSIONS The presence of additional space at the dentinal wall area did not affect secondary caries development as long as the enamel gap was small. However, with enamel gaps of approximately 500 μm, the presence of the additional gap space at the dentinal wall led to the development of smaller dentinal wall lesions at the deeper parts of the simulated cavity. In uniform gaps, the size of the interface was positively correlated with the size of the dentinal wall lesions.