Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) ; Processo FAPESP: 09/15170-3 ; Statement of problem. Intrinsic pigmentation is widely used to improve the esthetic features of denture base acrylic resins. However, acrylic resin may discolor over time.Purpose. The purpose of this study was to investigate the effect of intrinsic nanoparticle pigmentation on the color stability of acrylic resins.Material and methods. The acrylic resins, Onda Cryl, QC 20, Classico, and Lucitone, were evaluated. Twenty-one disk-shaped specimens (3 x 30 mm) were fabricated from each acrylic resin. Seven were colored with 3% Poli-Cor intrinsic pigment used to color denture base, 7 were colored with 7% pigment, and 7 were not pigmented. In addition, 7 specimens were fabricated containing only pigment. The specimens were thermally cycled 2000 times between 5 degrees C and 55 degrees C with a 30-second dwell time at each temperature. The specimen colors were measured with a spectrophotometer and evaluated with the CIE L*a*b* system before (B) and after thermal cycling (T). The pigment morphology was analyzed by using a scanning electron microscope (SEM) and energy-dispersive x-ray spectroscopy (EDS) techniques. The results were analyzed with 2-way nested ANOVA and Tukey HSD tests (alpha=.05).Results. Classico acrylic resin with and without pigment underwent the least color change, followed in order by Lucitone, Onda Cryl, and QC-20. The presence of pigments reduced the color change of the acrylic resins significantly (P