This article reports on an experiment undertaken in order to evaluate the effect of a program visualization tool for helping students to better understand the dynamics of object-oriented programs. The concrete tool used was BlueJ's debugger and object inspector. The study was done as a control-group experiment in an introductory programming course. The results of the experiment show that the students who used BlueJ's debugger did not perform statistically significantly better than the students not using it; both groups profited about the same amount from the exercises given in the experiment. We discuss possible reasons for and implications of this result. (Contains 1 footnote and 8 figures.)