The elimination of pathogenic microorganisms from the root canal system is one of the main points in order to have success in endodontic treatment. The objective of this study is to perform a randomized controlled clinical trial to compare the effectiveness of success in endodontic treatment of primary teeth when combined with photodynamic therapy. Will be selected 30 primary teeth of children aged between 3 and 6 years. The teeth will be divided into two groups: group I control, which will be applied the conventional endodontic treatment and group II to be held endodontic treatment with application of photodynamic therapy. In both groups will be made microbiological evaluations before and after endodontic treatment and clinical and radiographic evaluations will be conducted in the day, 1 month, 3 months and 6 months after treatment comparing the treatment performed in both groups.

Interventions All treatment procedures will be performed by a single, duly trained operator. Group I (control) - conventional endodontic treatment

1. Prophylaxis with pumice stone and water and antisepsis of region with 0.12% solution of chlorhexidine using moistened gauze
2. Anesthesia
3. Absolute isolation, if possible; in cases of considerable loss of remaining tooth, relative isolation using cotton rolls
4. Opening of crown with spherical diamond tip burs (bur size compatible with tooth) (KG Sorensen - Indústria e Comércio, SP, Brazil) and high speed Z endodontic drill under water and air refrigeration
5. Millimeter measurement of root canal to 1 mm short of radiographic apex
6. Insertion of three sterile paper cones with diameter compatible with canal for 30 seconds for initial collection of bacterial sample and immediate placement into brain heart infusion broth
7. Surgical chemical preparation with series of Kerr files appropriate for each case, using initial file and an additional two files of larger size, with irrigation and aspiration with 1% sodium hypochlorite (Milton's solution) and endo PTC (Fórmula & Ação) with each change of file
8. Insertion of three sterile paper cones for 30 seconds for second collection of bacterial sample and immediate placement into brain heart infusion broth
9. Filling of root canals with calcium hydroxide (Ultra-cal, Ultradent, Brazil); base of thin gutta-percha and filling with glass ionomer cement; restorative treatment performed in subsequent session

Group II - Conventional endodontic treatment + PDT

1. Prophylaxis with pumice stone and water and antisepsis of region with 0.12% solution of chlorhexidine using moistened gauze
2. Anesthesia
3. Absolute isolation, if possible; in cases of considerable loss of remaining tooth, relative isolation using cotton rolls
4. Opening of crown with spherical diamond tip burs (bur size compatible with tooth) (KG Sorensen - Indústria e Comércio, SP, Brazil) and high speed Z endodontic drill under water and air refrigeration
5. Millimeter measurement of root canal to 1 mm short of radiographic apex
6. Insertion of three sterile paper cones with diameter compatible with canal for 30 seconds for initial collection of bacterial sample and immediate placement into brain heart infusion broth
7. Insertion of sterile paper cone immersed in Chimiolux® methylene blue for three minutes; administration of wireless Therapy XT EC laser device (DMC - São Carlos, Brazil) after removal of cone; energy density: 4 J/cm², power: 100 mw; wavelength: 660 nm; exposure time: 40 seconds
8. Root canal irrigated with sterile saline solution and insertion of three sterile paper cones for 30 seconds for second collection of bacterial sample and immediate placement in brain heart infusion broth
9. Filling of root canals with calcium hydroxide (Ultra-cal, Ultradent, Brazil); base of thin gutta-percha and filling with glass ionomer cement; restorative treatment performed in subsequent session

Inclusion Criteria:

• Patients aged 3 to 6 years old with at least one anterior deciduous tooth with irreversible pulpitis or pulp necrosis in a position to be restored and at least 2/3 of the remaining root.
• Healthy patients and who did not antibiotic treatment in the previous three months.

Exclusion Criteria:

• Patients in poor health, with primary teeth with large root loss (+ 2/3 root), which has no internal pathological resorption, reconstruction is not possible, cases of retreatment and crypt involvement.