Due to the phenomenon of bacterial resistance, photodynamic therapy (PDT) has been studied as an alternative to the use of antibiotics on the periodontal treatment. However, the studies show several discrepancies between the parameters adopted for PDT, which makes it difficult to compare the results and measure their efficacy. The objective of this research was to investigate the tissue dosimetric parameters used in PDT for the treatment of periodontal disease. In order to achieve this objective, the scattering pattern of light in periodontal tissue, the concentration of methylene blue in the periodontal pockets and the number of viable microorganisms were evaluated. This clinical trial included 30 patients with chronic periodontitis according to the eligibility criteria. The patients were in periodontal treatment at the Dental Clinic at the University of Nove de Julho, where the research was carried out. Three incisors from each patient were selected for the experimental procedures. The pattern of light scattering was observed through intraoral photographs. A laser with emission of radiation with wavelength of 660 nm and 100 mW of power was used. The concentration of the photosensitizer (PS) in the periodontal pockets was evaluated by spectrometer before irradiation. Methylene blue was used as PS at the concentration of 100 μM in two different formulation and vehicles to evaluate the drug dimerization. Then, the procedures of irradiation of the PDT were carried out, with 3 irradiation times of 1, 3 and 5 min. The microbiological evaluation was performed from subgengival biofilms of the periodontal pockets of the experimental sites, before and immediately after the irradiation procedures.