There have been rapid increases in the incidence of colorectal cancer in Norway since the 1960s, and rates rank among the highest worldwide. The primary objectives are to describe trends in left-and right-sided colon cancer and rectal cancer by calendar period and birth cohort and to generate hypotheses as to the etiological factors in operation. Although the ageadjusted incidence rates of both colon and rectal cancer increased in Norway in both sexes up to the 1980s, subsite-and age-specific analyses reveal a deceleration in the rate of increase thereafter, apparent in the rates of both left-sided colon and rectal cancer. Overall trends in incidence of right-sided colon cancer continue to increase in both sexes. Rates in both left-and right-sided colon cancers have tended to stabilize or decrease among successive generations born after 1950, however, while incidence rates of rectal cancer appear to be increasing in recent generations. The all-ages rates are thus in keeping with the commonly reported ''left to right shift'' of colon cancer, although standardization masks important observations. The cohort patterns provide further evidence that factors earlier in life are important, and while the complex etiology makes interpretation difficult, modifications in diet, obesity and physical activity in Norway are likely among the drivers of the trends in one or more of the colorectal subsites examined. In summary, the recent downturn in the disease at younger ages provides some reason for optimism, although possible increases in rectal cancer among recent birth cohorts are of concern.The study of migrants and their swift acquisition of rates prevailing in (higher risk) countries of adoption-relative to rates in their country of origin 1-3 -has long implicated a vital role for the environment in colorectal cancer (CRC) progression. This has been strengthened during the last decades via the reporting of increasing incidence among a diverse number of cancer registry populations worldwide. 4-6 Some of the most rapid and steady rises in CRC incidence over the last half-century has been observed in Norway. [7][8][9] According to global estimates by country (for 2002), incidence rates in Norwegian women now rank second worldwide, while corresponding mortality rates are 5th and 10th highest in women and men, respectively. 10 At the national level, the disease is now more common than lung cancer, with only prostate and breast cancer more frequent in males and females, respectively.Intense research activity has led to the emergence of a number of putative pathways for CRC, and an underlying complexity that may explain the inherent difficulty in isolating specific risk factors capable of explaining the descriptive epidemiology. As a means to better elucidate the underlying etiological factors, there has been an increasing interest in epidemiological analyses that stratify by the major subsites of the large bowel, with meta-and pooled-analyses of such studies suggesting differential effects of lifestyle variables across the col...