Chromogenic extrinsic stains are common in children and frequently recur following conventional prophylaxis. Antimicrobial photodynamic therapy (aPDT) using methylene blue activated by red light has demonstrated antimicrobial activity against oral biofilm and may reduce stain recurrence. This pilot controlled clinical trial aims to compare conventional prophylaxis combined with methylene blue-mediated antimicrobial photodynamic therapy with conventional prophylaxis alone for preventing recurrent chromogenic extrinsic stains on deciduous teeth.