To simulate the thermoforming of a transparency, constitutive equations are proposed for the nonlinear viscoelastic behavior of poly(methyl methacrylate) near glass transition temperature, which include large deformations. In a first step, they are fitted on a set of uniaxial tension-relaxation tests at various strain levels and strain rates. In a second step, their implementation in a finite element code is performed. Finally, the thermoforming of a transparency at a constant and uniform temperature is simulated and compared with experimental results. POLYM. ENG. SCI., 50:2004-2012, 2010. (C) 2010 Society of Plastics Engineers