The aim of this paper was to investigate the effect of air temperature on labour productivity in telecommunication offices. The study was conducted as a case study in two call centres because the work in the call centres can be considered to represent typical activities in the telecommunication industry. The study design consisted of an observational approach and an intervention approach. In Call Centre 1, the productivity between two zones with temperature difference was compared. In Call Centre II, the intervention was conducted by installing cooling units to lower high temperature in the summer. Productivity was monitored both before and after the intervention, and it was measured as labour productivity by monitoring the number of telephone calls divided by the active work time. The indoor climate of both call centres was determined by measuring thermal climate and concentrations of relevant air pollutants as well as the acoustical environment and lighting levels. The study shows that productivity may fall by 5-7% at the elevated indoor temperatures.