The US Navy's Asbestos Medical Surveillance Program monitors civilian and military employees with current or historical exposure to asbestos. The prevalence of definite radiologic parenchymal abnormalities (ILO category greater than or equal to 1/0) was 3.10 percent for 32,233 smokers vs 1.09 percent for 13,414 nonsmokers. The age-adjusted prevalence ratio was 2.23. The difference in prevalence between smokers and nonsmokers persisted in all age groups studied and was greatest among the oldest employees. These findings in a large population suggest that parenchymal opacifications alone will not reliably differentiate between the impacts of asbestos exposure and smoking in patients who face both risks.