OBJECTIVE Lung cancer is a common malignancy among people with AIDS (PWA). Lung cancer risk was compared between PWA and the general population and its relationship with immunosuppression was assessed. The likelihood that excess risk is explained by a high prevalence of smoking was also investigated. METHODS Records on adolescent and adult PWA (N = 397 927) were linked with cancer registries in 11 US regions. Cancer risk was assessed for the period 60 months before to 60 months after AIDS onset, with specific emphasis on the period 4-27 months after onset. Observed incidence was compared with general population rates and rates from a lung cancer prediction model for smokers. RESULTS Compared with the general population, lung cancer risk among PWA was elevated overall [n = 1489 cases; standardized incidence ratio (SIR), 3.8; 95% confidence interval (CI), 3.6-4.1] and in the 4-27 months after AIDS (n = 393 cases; SIR, 2.9; 95% CI, 2.6-3.2). In the 4-27 months after AIDS, risk was significantly elevated for all demographic subgroups, and was especially high among young PWA (SIRs for ages 15-29 years, 10.4; 30-39 years, 6.3; 40-49 years, 3.7). Lung cancers generally presented at an advanced stage. Risk was not associated with CD4 cell counts at AIDS (Ptrend = 0.36). Under plausible smoking assumptions, observed incidence was significantly higher than predicted among 40-49 and 50-59-year-old men with AIDS (observed/predicted = 5.03 and 1.43, respectively) and 40-49-year-old women with AIDS (observed/predicted = 1.88), but not among older PWA. CONCLUSIONS Lung cancer risk was substantially elevated among PWA. Smoking could not entirely account for the observed elevation, especially among younger adults, suggesting a role for additional co-factors.