A cross-sectional survey was conducted from November 1994 to March 1995 in 12 universities selected from three Chinese cities. Eight of the universities (two medical and six non-medical) were in Beijing, the capital, in northern China. The rest were in Wenzhou and Hangzhou, mid-size cities in south-eastern China. Although a formal multi-stage sampling procedure was not used, systematic steps were taken to ensure that multiple schools were chosen. Additionally, classes were chosen in such a manner that similar numbers of medical and non-medical students were sampled, and the relative proportions of these two groups were equivalent between the northern and southern cities.