- The problem of information (state) reconstruction from available measurements under HIV-1 infection will be addressed. First, the HIV-1 nonlinear model is recalled, and its dynamic properties are detailed. Next, a reduced nonlinear observer is constructed based on a linear coordinates transformation. The transformed system has now nonlinear terms that depend on the actual measurements. Simulation results are illustrated that show the convergence of the estimation despite initial conditions and initialization time. Moreover, the estimation robustness was also investigated through a random evaluation. © 2007 IEEE.