The purpose of this study was to examine changes in subsite distribution and incidence of colorectal cancer within different age groups. Registration of colorectal cancer by the National Cancer Registry of New Zealand approached 100 percent by 1974. The present study was based on 15,395 individuals aged 25 years and over and registered for colorectal cancer between 1974 and 1983. Subsite distribution (right colon, left colon, rectum) for different age groups (25-49, 50-69, 70+ years) was significantly skewed, with an excess of right colonic cancer in individuals aged 25-49 years and 70+ years. This right colonic excess was accompanied by a relative reduction in left colonic cancer. Age adjusted incidence rates for the periods 1974-78 and 1979-83 were compared and stratified by age group and subsite. Incidence rates increased in all subsites in individuals aged 50+ years. This was particularly evident for right sided cancer in the elderly of both sexes. There was a marked reduction in the incidence of left colonic cancer and rectal cancer in individuals under 50 years. In contrast, the incidence of right colonic cancer remained relatively stable in young individuals. Time trend studies indicate that the skewed subsite distribution of large bowel cancer in different age groups may increase with time and is probably due to varying etiological factors acting on different cohorts.