Through 2004, five cancer registries in Thailand have collected data for more than ten years. Three-year cancer incidence in Thailand covering the years 1989-1997 has been regularly reported in three volumes of 'Cancer in Thailand. Since the data for the last decade of the 20th century have been collected, the trends in incidence of some cancer sites were analyzed. Data sources were registry data from Chiang Mai, Lampang, Khon Kaen, Bangkok, and Songkhla, which are representative of the four major geographic regions of Thailand. The data drawn in 2002 covered the years 1989 to 1997 for Bangkok, the other four registries drew data from 1989 to 2000. The population denominators were estimated from the two censuses in 1990 and 2000. Only cancers of the liver, lung, colon-rectum, female breast, uterine cervix, and all cancer sites were analyzed since cancers of these sites may have major public health impacts. Age-specific incidence rates of different 5-year age groups were projected through the period 2007-2009 using a linear regression model if the rates were increasing, and a log-linear model to prevent prediction of a negative rate if the rates were decreasing. During the past decade, colorectal and breast cancers showed a statistical significant increasing trend, while the trend was generally stable for cancer of other sites. The number of new cancer cases of all sites is expected to be approximately 125,000 by the year 2008, compared with 81,000 in 1999. However, the accuracy of projections depends very much on the quality of the cancer registries' data. The Bangkok registry significantly improved case ascertainment in recent years, while the Chiang Mai registry had a consistent drop in incidence of cancer at many sites. In-depth investigation of some cancer sites and age period cohort modeling are required for better understanding of cancer trends in Thailand.