Morphological changes of Er:YAG irradiated enamel represented mild to severe damages. Conditions employed in this study are not recommended for deciduous caries prevention. Er:YAG energy density influenced chemical changes in enamel to enhance its structure. Acid dissolution removed fluoride from enamel surface.

Erbium-doped yttrium aluminium garnet (Er:YAG) lasers and fluoride have been used as preventive treatments to combat permanent tooth decay; however, little is known about deciduous teeth, which are more prone to caries. The purpose of this study was to assess acid resistance, roughness, and microhardness induced by an Er:YAG laser, fluoride application and a combined treatment on deciduous enamel. Seventy samples of deciduous enamel were randomly assigned to seven groups (n = 10): a negative control group [G1_C (−)], a positive control group [G2_C (+)], a group treated with acidulated phosphate fluoride (G3_F), a group treated with an Er:YAG laser (G4_L7.5 and G5_L12.7) and one tretated with an Er:YAG laser plus fluoride (G6_L7.5+F and G7_L12.7+F). The samples were then placed in a lactic acid solution for 24 h; the calcium and phosphate release was measured by atomic absorption spectroscopy and ultraviolet-visible spectroscopy (UV–vis). The surface roughness and microhardness were then analysed using a stylus profilometer and a static microhardness tester, respectively. In relation to the calcium release, G3_F presented values statistically lower than all others groups (p < 0.05). Group G7_L12.7+F had lower phosphorus release values than all other groups (p < 0.05). Group G3_F (0.24 ± 0.14 µm) had lower roughness values than groups G6_L7.5+F and G7_L12.7+F. Group G1_C(−) presented statistically significant differences in comparison to the rest of the groups (p < 0.05). The laser treatment and the combined treatment (laser plus fluoride) were not more effective than the fluoride treatment alone at improving acid resistance of deciduous enamel. However, the combined treatment diminished phosphorous release and increased the surface roughness of deciduous enamel. Finally, the acid dissolution decreased the microhardness of deciduous enamel regardless of the preventive protocol (fluoride, laser, or laser plus fluoride treatment).