Abstract This paper compares the remineralization of human natural caries and artificial caries-like dentin lesions treated with a novel whisker-reinforced experimental composite resin (ART composite) with a resin-modified glass ionomer cement (RM-GIC) as control. Ten molars with moderate natural dentin caries were prepared (N). Artificial caries-like dentin lesions were prepared in occlusal dentin of 10 caries-free molars and demineralized at pH 4.3 for 48 h (A). The cavities were restored with ART composite or RM-GIC. All restored teeth were sliced into 120 μm sections. Transverse microradiography combined with digital image analysis was performed to analyze the change in mineral density at the same position in the specimens before and after 4 and 8 weeks remineralization/demineralization treatment. The mean percent remineralization ± standard deviation after 4 and 8 weeks are: N with ART composite, 27 ± 9 and 46 ± 14, respectively; N with RM-GIC, 18 ± 6 and 36 ± 11, respectively; A with ART composite, 48 ± 9 and 66 ± 11, respectively; A with RM-GIC, 50 ± 13 and 62 ± 11, respectively. There was a significant difference between the ART composite and RM-GIC for the remineralization of natural caries ( P < 0.05). For both restoratives there were significant differences between the remineralization of natural and artificial caries ( P < 0.001). The ART composite and RM-GIC remineralized natural and artificial caries differently, most likely due to differences in the microstructure and composition of the caries dentin.