Outbreaks of tuberculosis (TB), which are of a type rarely experienced in the past, have recently increased in Japan. An example of such an outbreak, which occurred in a modern building with fixed sash windows, will be described. The occurrence of four TB cases, which had been found during the period between May 1979 and June 1980 among company employees working in the same building in downtown Tokyo, motivated the conduct of epidemiological and environmental surveys on this episode. The first case, a 36-year-old male with a positive smear and cavities in both lungs, was considered to have been in an infectious state for about one year before his admission to a TB hospital in June 1979. Follow-up investigation of 99 contacts until March 1982 revealed the occurrence of 16 secondary cases. The secondary attack rate was highest among those working on the same floor of the building as this first case, but no case was found among employees of another company that occupied this building. The desks of secondary cases were aggregated near and around that of the first case. Although this building had central air-conditioning, ventilation was often closed for energy conservation purposes. This resulted in the increase of carbon dioxide concentration in the air up to a level of more than 1,000 ppm during working hours and to as high as 2,000 ppm when the ventilation was closed. It was thus concluded that the indoor infection of TB in this episode was attributable to the insufficient ventilation in the building.