The period-age contours of rectum and colon cancer show a concurrent rise and fall among old and young age groups, respectively (graphs not shown). In general, such divergent patterns among period-age contours of consecutive age groups are highly suggestive of an underlying birth-cohort phenomenon.3 To delineate such a cohort phenomenon, the Figure shows age-specific death rates of rectum cancer (upper panels) and colon cancer (lower panels) replotted against the period of birth as cohort-age contours. In both cancer types, the individual cohort-age contours align in patterns that resemble hyperbolas with an initial rise and subsequent decline associated with consecutive periods of birth. The oldest age groups participated in the initial rise and parts of the subsequent decline. The younger age groups contributed mostly to the recent decline. The location of the peak associated with the highest mortality from rectum or colon cancer appeared to occur before the turn of the century (1900s). Overall, the alignment of all the individual cohort-age contours to form a single overarching pattern suggests that risk of death from colon cancer was strongly associated with the period of birth.