Biorhythm theory which became popular with the general public in the late 1960s held that three different biorhythm cycles influenced three different general aspects of human behavior. There was a 23-day cycle which influenced physical aspects of behavior. A 28-day cycle influenced emotions and a 33-day cycle influenced intellectual functions. Further, according to the theory, these three cycles started at birth and progressed, sinusoidally, throughout life varying not a bit with environmental or physiological factors. This paper reviews 134 studies of biorhythm theory, both published and unpublished. 35 of those reported some support for biorhythm theory. Careful examination of these studies identified methodological and statistical errors that account for the claimed findings supporting the theory. Another 99 studies, covering many different variables, yielded no support. The conclusion is that biorhythm theory is not valid.