Since the cessation of vaccination for whooping cough in Okayama Prefecture in 1973, the tendency of the outbreak of whooping cough-patients is being watched with keen interest in the field of public health. From the outbreak of patients in four monitering hospitals in Okayama Prefecture, the authors conducted analysis of the tendency of the outbreak of whooping cough-patients by analysis of time series. The results may be summarized as follows; 1. The tendency of the outbreak of whooping cough-patients by month analyzed by average monthly indicator in 1976 and 1977. 1) The outbreak of patients in O-Hospital located in the southeastern part of Okayama Prefecture increased gradually from January and reached a smaller peak in April and reached a larger peak from July to September and decreased later. In comparing the outbreak of patients in each of other hospitals with that in O-Hospital, the distinctive features could be recognized as follows. In K-Hospital located in the southwestern part of Okayama Prefecture, a larger peak was observed in April and May, and a moderate peak was observed from August to October. In Y-Hospital located in the northwestern part, no peak was recognized in April, and a larger peak was recognized in November as well as from July to September. In T-Hospital located in the northeastern part, a moderate peak was recognized in December. 2) From January to June, the tendency of the outbreak of patients in southern part was earlier than that in northern part. From July to October, monthly indicators in both northern and southern parts were high. In November and December, monthly indicator in southern part dicreased below 1.0, but that in northern part did not dicrease below 1.0. Data indicate that differences in the tendency of the outbreak of patients by month between northern and southern parts of Okayama Prefecture were recognized. 2. The tendency of the outbreak of whooping cough-patients by year analyzed by average monthly number of patients in 1976 and 1977 and average monthly estimated number of patients in 1978. 1) The number of patients in each of four hospitals was greater in 1977 than in 1976. 2) Statistically, it is forecarsted that the number of patients in O-Hospital would decrease more remarkably, and that in K-Hospital and Y-Hospital would increase gradually, and that in T-Hospital would increase more remarkably in 1978 than in 1977. Data indicate that differences in the tendency of the outbreak of patients by year among four districts were recogniized. 3. The estimated number of patients in 1978. The estimated number of patients in O-Hospital is calculated as 52.9, and that in K-Hospital is calculated as 312.6, and that in Y-Hospital is calculated as 91.1. and that in T-Hospital is calculated as 336.3. 4. Discussion in the field of public health. 1) The authors discussed in respect to the tendency of the outbreak of whooping cough-patients in all of Okayama Prefecture and methods to grasp it. 2) Data of the tendency of the outbreak of whooping cough-patients by analysis of time series will be an important factor for taking countermeasures in the field of public health. 3) It is necessary for the party concerned to conduct surveillance and take countermeasures for whooping cough in each district. 4) It is necessary for the party concerned to analyze epidemiological factors as well as to conduct surveillance of the outbreak of whooping cough-patients.