Analytic forms of the changes of the absorption and refractive index associated with intersubband optical transitions in a quantum box are obtained by using the compact density matrix approach. Numerical results are presented for a typical AlGaAs/GaAs quantum box system. The linear, third-order nonlinear, and total refractive-index and absorption changes are examined as a function of the incident optical intensity. The results obtained show that the incident optical intensity has a great effect on the total absorption and refractive-index changes. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.