The authors present the classification of biorhythms from the cybernetic aspects. They utilize this approach in describing and explaining the clinical course of periodic diseases in general, and of primary affective disorders in particular. In more complex organisms there exists a great number of single subsystems and their interactions, and therefore more possibilities arise for the development of natural biorhythms. If the number of single subsystems is ‘n’, the number of possible interactions may reach the maximum value of’n!’. In more simple organisms the pseudobiorhythms may develop. This study focuses on the relationship of proper biorhythms to the recurrent course of illness, especially to that of primary affective disorders (PAD). Based on Fourier’s decomposition of the periodical course into single ones, the authors hypothetically explain the peculiarity of the periodic course of PAD. In addition to sine form courses, the recurrences may originate from superimpositions of courses which originate from nonlinearity of single subsystems and are not harmonic. In real biosystems, such courses are undoubtedly more frequent than purely harmonic courses. This observation does not contradict the applied mathematical model, because each of these nonharmonic periodical courses may be decomposed into the harmonic ones. The proposed hypothesis presents an alternative or a supplementary model to the prevailing hypothesis which explains the course of PAD by the desynchronization of circadian rhythms controlled by special bioos