Circadian rhythmicity is a prominent feature of many important biologic functions. Failure to take such variation into account when performing assays may lead to increased assay variation or, most importantly, an erroneous result. Circadian rhythms of immunologic relevance include variation in the number of circulating T cells (1 ), the autologous mixed lymphocyte reaction (2 ), the phagocytic index (3 ), and urinary neopterin secretion (4 ).