The high morbidity of common colds means that their economic importance is considerable, with colds causing more loss of productivity than any other infection. As no effective prophylaxis is available, this trial was to test the hypothesis that sauna bathing can reduce the incidence of common colds. Twenty-five volunteers were submitted to sauna bathing, with 25 controls abstaining from this or comparable procedures. In both groups the frequency, duration and severity of common colds were recorded for six months. There were significantly fewer episodes of common cold in the sauna group. This was found particularly during the last three months of the study period when the incidence was roughly halved compared to controls. The mean duration and average severity of common colds did not differ significantly between the groups. It is concluded that regular sauna bathing probably reduces the incidence of common colds, but further studies are needed to prove this.