In a study of client variables related to length of stay in two types of drug-free day treatment programs, results varied according to program structure and sex of the clients. In the more loosely structured Youth Centers, in which the programs differed from center to center and the populations were heterogenous, there were no clear-cut results. However, within the more tightly structured Ambulatory Treatment Units, the programs and the populations were more homogeneous, and the results showed a significant pattern related to length of stay, but for males only.