We studied the integrity of working memory operations in 38 HIV-seropositive and 20 seronegative drug users, using a modified version of the Tower of London task. This new task, the Tower of London-Working Memory version (TOL-WM), includes a delayed-response component in addition to the planning required for successful performance of the standard TOL. Symptomatic HIV-seropositive participants solved significantly fewer TOL-WM problems compared to matched seronegative controls. However, seropositive and seronegative subjects showed similar overall levels of planning efficiency, suggesting that the TOL-WM deficit may be associated primarily with failure to encode or maintain an adequate online memory representation. The results of this study confirm our previous report of a possible working memory deficit in HIV-1 infection and suggest that measures of working memory have particular utility in the evaluation of HIV-related cognitive deficits.