Rhythm characteristics in the about-daily (circadian), about-monthly (circatrigintan), and about-yearly (circannual) frequency ranges were assessed for plasma PRL, which is possibly involved in the development of human breast cancer. Clinically healthy subjects in Minnesota, USA, and Kyushu, Japan, were sampled around the clock once in each season. Possible differences that could reflect the large difference in breast cancer incidence in these two geographic locations were investigated. If sampling techniques as extensive and systematic as those used in this study are employed, Japanese women have a slightly higher plasma PRL concentration than women in Minnesota, but these differences depend largely on nightly values during the winter. A prominent circadian rhythm characterizes plasma PRL in most subjects investigated. In winter and spring, Japanese women have a much larger circadian amplitude than Minnesotan women, mostly as a result of markedly higher concentrations during nightly rest and/or sleep. The larger circadian amplitude in winter and spring of the Kyushuans is the result of a circannual cycle, which, with the sampling schedule used, is not detected in Minnesotans. Rhythm parameters also differed as a function of age. Certain rhythm characteristics correlated with breast cancer risk. This result should be checked on much broader populations. The task is facilitated by sampling requirements based costeffectively on fewer yet pertinent sampling times, selected on the basis of the time-specified tolerance intervals here documented.