In studying the periodicity of mania onsets, cycle-oriented diagrams were made of the clinical course from 257 manic episodes analyzed retrospectively in 34 bipolar I manic-depressive patients for a period of about 5 years. Using these diagrams, the frequent period of mania onsets located in one-quarter of the follow-up period was pre-estimated, and the accordance ratio during a 25 month follow-up period was analyzed. The accordance ratio in all subjects was 39% (11/28) for the first episode and 35% (7/20) for the second episode. These ratios were not significantly different from the expected level (25%). The number of subjects was limited to 11 patients (10 rapid and 1 non-rapid cyclers) whose number of episodes used for the determination of the index cycle was eight or more. The accordance ratio was 64% (7/11) and 60% (6/10) for the first and second episodes, respectively. Their levels were significantly higher than those expected. Periodicity of mania onsets existed at least in rapid cyclers with abundant past data.