The present study investigates whether it is possible to achieve equally satisfactory results between 37.5% hydrogen peroxide gel and 6% gel for teeth bleaching. A prospective, double-blind, randomized clinical trial was carried out. A total of 33 patients were selected from the clinic of the Faculty of Dentistry at the University of Chile. The patients included men and women over 18 years old without prior tooth whitening treatments, tooth decay, or restorations of the upper front teeth. The patients had tooth colors of A3 or less according to the Vita Classical scale, which was determined with a Vita Easy® Shade spectrophotometer. The study was carried out with a "split-mouth" design. One side of each mouth was randomly treated with 37.5% hydrogen peroxide, and the other side was bleached with 6% hydrogen peroxide. Each group received 3 to 12 minutes of treatment with the respective gel applications. Two sessions of bleaching were carried out each week. Color was assessed at 7 different sessions for 3 months. The spectrophotometer was used to measure the total variation of color (ΔE) between the baseline (session 1) and different measurement times (sessions 2-7). We compared ΔE for both agents using the Mann-Whitney test.