OBJECTIVE To evaluate the relationship between exposure to gaseous air pollutants (NO2 and SO2) and daily hospital admissions for respiratory diseases. METHODS After controlling long time trend, the "day of week" effect and confounding factors of meteorology, a semi-parametric generalized additive model (GAM) was used to analyze the exposure-effect relationship between gaseous air pollutants and daily respiratory hospital visits from 2001 to 2005 in Lanzhou city. RESULTS Both NO2 and SO2 have positive relationships with residents' hospital visits for respiratory complaints. In single-pollutant models, the best fits for NO2 and SO2 were one day after (Lag1) and the same day visit (Lag0). With an IQR concentration increase in NO2 (Lag1) and SO2 (Lag0), the relative risks of daily hospital admissions for respiratory diseases were 1.060 (95% CI 1.046-1.074) and 1.048 (95% CI 1.031-1.065), respectively. The susceptible populations were female and the elderly. The health effect of gaseous air pollution was more obvious in autumn and winter. CONCLUSIONS The ambient NO2 and SO2 concentrations have an adverse effect on daily hospital visits for respiratory diseases of residents in Lanzhou.