A population-based cancer registry was established in Qidong, Jiangsu Province, China, in 1972, and the trends in incidence rates of the major cancer sites have been analyzed for a 25-year period, 1978-2002. Five-year age-specific rates, crude incidence rates, world age-standardized rates (ASR), percent change (PC) and annual percent change (APC) were calculated using annual data on population size, and estimates of its age structure. The indices of histological verification of diagnosis, death certificate only and proportion of mortality to incidence were employed for assessing the registration quality. A total of 51,933 incident cases of cancer were registered in Qidong from years 1978 to 2002, with a male-to-female sex ratio of 1.9:1. Crude incidence increased markedly over the 25-year period (PC and APC of +55.6% and +2.1%, respectively), but ASR showed a slight decrease (-0.4% in males, and -0.3% in females), indicating that the major part of this is due to population ageing. The leading cancer sites in rank were liver (average ASR = 50.8 per 100,000), stomach (26.7), lung (22.7), colon-rectum (8.9), oesophagus (7.4) and breast (5.4). Cancers of liver, lung, colon-rectum and female breast all showed increases in incidence during the study period, with APCs (ASR) of +0.1%, +1.7% and +1.4% for males, and +0.2%, +0.9%, +1.9% and +1.1% for females, while the cancers of stomach (APC: -3.2% in male, and --2.4% in female) and cervix (APC: -4.7%) showed notable declines. Examination of age-specific rates showed declining trends in the younger generations for liver cancer, but increases for cervix cancer. The results underline the increasing importance of cancer as a cause of mortality and morbidity in a population that is ageing and undergoing profound changes in socioeconomic development and lifestyle. The cancers of high lethality that have been common in the Chinese population (liver, stomach, oesophagus) are showing some evidence of decline, at least in younger generations, but they remain major problems. At the same time, the cancers associated with economically "developed" societies -- lung, colon-rectum and female breast -- are showing increases. The population-based cancer registry is an indispensable tool for providing data for planning and evaluation of programmes for cancer control in all societies.