Arsenic and prior lung diseases have been shown to increase lung cancer risk; however, little is known about their joint effects. The aim of our study was to analyze the joint effects of inhaled arsenic and prior lung diseases on lung cancer risk within a occupational cohort. The interactions of prior lung diseases and inhaled arsenic were analyzed based on multiplicative and additive scales in the Cox proportional hazards model. Compared with low arsenic exposure and no history of asthma, the hazard ratios (HRs) of high arsenic exposure with asthma, high arsenic exposure without asthma and low arsenic exposure with asthma were 2.61 (95% CI: 1.71-4.00), 2.60 (95% CI: 1.93-3.51) and 2.49 (95% CI: 1.53-4.06), respectively. Based on the multiplicative scale in the Cox proportional hazards model, the HR of the interaction of asthma and arsenic on lung cancer risk was 0.45 (95% CI: 0.25-0.80). Based on the additive scale, the relative excess risk due to interaction between asthma and arsenic was -1.41 (95% CI: -2.81 to -0.02). Our study provides strong evidence that arsenic exposure is associated with lung cancer risk. A significant negative interaction between asthma and arsenic on lung cancer risk is observed.