Objective: In this split-mouth clinical trial, we evaluated the clinical benefits of low-level laser therapy (LLLT) as an adjunct to nonsurgical periodontal treatment in patients with type 2 diabetes mellitus (DM). Background data: The impaired wound healing seen in diabetic patients may affect the results of periodontal treatment and may require an additional approach. Materials and methods: In total, 22 chronic periodontitis patients with type 2 DM were included. Applying a split-mouth design, two quadrants were treated with only scaling and root planing (SRP) as the control and those in the other two were treated with SRP + LLLT as the test sites in each patient. An 808nm GaAlAs diode laser was performed in the test sites at the energy density of 4.46 J/cm(2) on days 1, 2, and 7 after SRP. Plaque index (PI), probing depth (PD), bleeding on probing (BOP), and clinical attachment level were measured at baseline and again at 1 and 3 months after treatment. Deep periodontal pockets (PD >= 4mm) were evaluated separately. Results: Test sites showed significant improvement in PI and BOP in deep pockets at the 1-month follow-up period (p < 0.001 and < 0.001, respectively), whereas no difference was found between the control and the test sites in other periodontal parameters. Conclusions: LLLT during periodontal treatment offered minimal short-term additional benefit in deep pocket healing in patients with type 2 DM.