A procedure for empirical initialization of the trait. estimator in adaptive testing is proposed that is based on the statistical relation between and background variables known prior to test administration. The relation is modeled using a two-parameter version of a logistic item response theory model with manifest predictors discussed in Zwinderman (1991). Equations are provided that are necessary for estimating the parameters from an incomplete sample of response data and data on background variables. The procedure is illustrated for an adaptive version of a test from the Dutch General Aptitude Test Battery, with response time on a prior test as a background variable.