In recent years, tuberculosis (tbc) epidemics have been increasing, and have become a social problem in Japan. This study was carried out to clarify the current risk factors on tbc epidemics. The original reports of a total of 254 tbc patients whose contacts were examined for possible tbc infection (extraordinary health examinations) during 1992-97, were investigated and compared with all new pulmonary tbc cases registered during the same period in Aichi prefecture. In addition, from registration cards in each public health center, the risk factors for 19 cases of both tbc epidemics (infecting more than 20 contacts) and microepidemics infecting 10-19 contacts were further examined. The results obtained were as follows; 1. Extraordinary health examinations were carried out in 3.2% of total pulmonary tbc cases, and were carried out at higher rates among contacts with younger patients and with those having severer tbc by radiographical (p < 0.01) and bacteriological findings (p < 0.01). 2. 11 tbc epidemics (3.4%) and 34 microepidemics (14.3%) were identified among 254 examinations over 6 years. They were frequently observed in groups with abundant bacilli discharge and long lasting cough, and also in young age groups. 3. All of the 19 cases causing epidemics or microepidemics had multiple risk factors; the main factors being late diagnosis, inadequate health managements in schools and offices and poor quality of patients' living environments. Therefore, it is necessary for public health centers to work more closely with schools and industrial circles for the prevention of tbc epidemics.