To investigate whether circadian or any other temporal pattern(s) exist in the occurrence of acute myocardial infarction (AMI) in a Chinese population, we analyzed 428 patients with confirmed AMI for temporal patterns of AMI occurrence. The patients admitted to the Affiliated Hospital of Shandong Medical University during 1991-95 were from Jinan, the capital city of Shandong Province of China, which has a population of 2.5 million. The chi-square test for goodness of fit was used to test the difference among the frequencies of AMI occurrence in 4 equal intervals (O1.00-07.00 h, 07.00-13.00 h, 13.00-19.00 h, 19.00-01.00 h) during the day and among those on 7 days during the week. The results showed that AMI occurrence exhibited significant circadian (p<0.001) and septadian (day of the week) (p=0.046) periodicity, with a peak at O1.00-07.00 h and a trough at 13.00-19.00 h during the day, and a peak on Saturday and a trough on Wednesday during the week. The peak to trough ratio of risk was 2.7 during the day and 2.1 during the week. It is concluded that there were circadian and septadian biorhythms in AMI occurrence in the Chinese population and that these were different from those observed in Western populations. Further investigation of the underlying mechanisms may shed further light on the trigger mechanisms of AMI and thus be helpful in the prevention and treatment of AMI. (Jpn Circ J 1998; 62: 190 - 192)