The aim of the study was to investigate the effects of elevated summer indoor temperatures onperceived comfort, symptoms and subjectively assessed productivity in an office building.The measurements during spring 2002 showed that indoor environment was at the normalFinnish office building level. The mean room temperature during the 2-week measurementperiod was 22C. An indoor air questionnaire (MM-40-FIN) including additional questionsabout work efficiency was presented to the employees during the measurements. Theperceived indoor environment was at the normal Finnish office level. During summer 2002,another series of temperature measurements and questionnaires were carried out. When thetemperature rose over 25C, central nervous system symptoms and the dissatisfaction of IAQincreased. Self-estimated work efficiency decreased statistically significantly among theemployees, with the temperature. The number of employees reporting reduced workefficiency increased by about 40% when the indoor temperature rose from 24 to 29C.