The purpose of this study was to assess the possible impact of rising prevalence of obesity in US adults (since the late 1970s) on incidence rates (1973-1999) for cancers strongly associated with obesity, and to estimate the number of incident cancers (in 2002) potentially attributable to obesity. Data from a group of high-quality population-based cancer registries, regarded as generally representative of the unknown cancer rates in the entire US, were used to obtain average annual age-standardized incidence rates (ASIRs) per 100,000 from 1973 to 1975 through 1997-1999 for selected obesity-related cancers. Temporal increases in ASIRs were evident during the entire period for kidney and adenocarcinomas of the esophagus and gastric cardia, while during the 1990's the decline in ASIRs for adenocarcinoma of the uterine corpus was reversed and the ASIRs for breast cancer continued to increase for age 50+ years, suggesting a potential impact of rising obesity prevalence rates. An estimated 41,383 new cancers (about 3.2% of all cancers) diagnosed in the US in 2002 may be potentially attributable to obesity. Further analytic epidemiologic studies are needed to assess the risks of other cancers in relation to both obesity and body fat distribution.