Objective To analyze the current prevalence of metabolic syndrome (MS) and its trend in the past 5 years in China. Methods In 2004-2005, cross-sectional sampling surveys were conducted in 13 research populations from southern and northern parts, urban and rural areas in China, with internationally standardized criteria and measurement. 1000-2000 participants for each group, even number of man and women, were examined. Out of them, there were 13 732 participants aged 35-59 years. Comparison was made for 10 populations with comparable data in 1998 to observe the trend in the past 5 years. MS was defined according to Chinese Medical Association Diabetes Branch definition set in 2004. Results The prevalence of MS was 12.1%. The adjusted prevalence was higher in men (13.9%) than in women (8.5%), higher in residents from the northern (15.0%) than the southern (6.8%) and from urban (11.3%) than rural (10.1%). Out of the participants with MS, 60.4% were with obesity, high blood pressure or with dyslipidemia. Comparing with data in 1998, the adjusted prevalence rates of MS in 2004--2005 had a significantly increase (10.0% vs. 7.2%, P<0.05), in men than in women, in residents from the northern part than in the south (P<0.05). Regardless the geographic areas as from the north or south, from urban or rural, the mean levels of triglyceride, body mass index and waist circumference were higher in 2004 - 2005 than that in 1998. Conclusion The prevalence of MS was increasing in middle aged Chinese population. Prevention and treatment of obesity, high blood pressure and dyslipidemia were effective ways to control MS.