This study evaluated the effect of a CPP-containing toothpaste and compared it with fluoride-containing toothpastes on remineralization of caries-like lesions in primary teeth enamel, using polarized light microscopy. Forty-eight sound primary incisors were coated with nail varnish, leaving two 1x1 mm windows before being placed in a demineralizing solution for 4 days. After demineralization, all the specimens were coated with nail varnish over one window and were randomly assigned to 4 groups (A to D; n = 12). Group A teeth were exposed to distilled water. Group B teeth were exposed to a CPP-containing toothpaste (Hi Herb). Group C teeth were exposed to a 260 ppm fluoride-containing toothpaste (Smile baby toothgel). Group D teeth were exposed to a 500 ppm fluoride-containing toothpaste (Oralmed Kid). Polarized light microscopy was used to evaluate lesion depth, before and after a 7-day pH cycle. Lesion depth was measured using a computerized method with the Image-Pro Plus program. Differences in mean lesion depth within groups and between groups were analysed using the paired t-test, Kruskal-Wallis test and Mann-Whitney U test at a 95% level of confidence. Mean lesion depths in Groups A, B, C and D significantly increased by 110.1, 36.1, 40.2 and 18.2%, respectively. The mean lesion depths for all the toothpaste groups (B, C and D) were significantly different from the control group (A). Comparisons made among treatment groups showed Group D was significantly different from Groups B and C. All toothpastes were effective for inhibiting progression of carious lesions. However, a 500 ppm fluoride-containing toothpaste inhibited lesion progression better than a CPP-containing toothpaste and a 260 ppm fluoride-containing toothpaste.