Abstract To study the associations between dorm environment and occupants' health, a nested case-control study on 348 college students was carried out in 2006-2007 at Tianjin University, China. Two hundred and twenty-three dorm rooms where the 'cases' and 'controls' resided were inspected. Measured variables were ventilation rate, air temperature, and relative humidity indoors. Allergic symptoms in the last 12 months were self-reported by occupants. Adjusted odds ratios (AORs) of a 'localized moldy smell/moisture indicator' in 'special places' (e.g., in a room corner or close to the radiator under the window) for wheezing was 3.56 [95% Confident Interval (CI): 1.56-8.14] and for rhinitis 2.81 (95% CI: 1.32-5.97). The AOR of a low air change rate (below the median value of 0.7/h) for wheezing was 2.28 (95% CI: 1.38-3.75) and for dry cough 2.26 (95% CI: 1.08-4.75). The prevalence of students with allergic symptoms in dorm rooms decreased with increasing ventilation rate. The combination of a 'localized moldy/moisture indicator' and a low air change rate significantly increased the AOR of case status to 13.35 (95% CI: 3.73-47.83), compared to the reference condition with no-dampness and high ventilation rate (above the median). This supports the hypothesis that ventilation rate is an effect modifier for moisture problems and indoor pollutants. PRACTICAL IMPLICATIONS: Dorm rooms, a kind of residential environment for students, may be more polluted than the home environment. This is especially the case in developing countries like China, where dorms tend to be more crowded. In dorms, a low ventilation rate is a risk factor for asthma and allergy. Sufficient fresh outdoor air should be provided to students' dormitories by controlled ventilation. Mechanical ventilation system are often needed in regions such as north China, as the buildings are now 'tight' and opening of windows is not a solution during the cold winter.