Current clinical data show a strong correlation between time of day and illness manifestation or immune activity. For example, symptoms of rheumatoid arthritis occur during the morning (Katz et al. 2002) most asthma attacks during the night (Reinberg 2006), and the effects of immunization also change with daytime (Langlois et al. 1995). Taken together, these reports suggest a strong regulation exerted by the circadian clock on the immune system, which will be reviewed in this article. Moreover, clock-controlled rhythms in several variables exert a feedback regulation on the circadian oscillator itself, a mechanism that we shall also consider in this paper.Sociedad Argentina de Fisiología