In a search for an effective prophylaxis against common colds, 25 volunteers were treated regularly with a standardised hydrotherapy programme for half a year, while 25 matched untreated persons served as controls. The frequency, duration and strength of colds were recorded. Resultsshow that there is a significant reduction in frequency of colds in the treated group. Similarly, this group seemed also to be affected less severely. The duration of common colds was shorter in the treated group which, however, failed to reach the level of significance. The prophylactic effects took about three months to become apparent. The underlying mechanism, by which the effect is mediated, is speculative at present. It is suggested that regular hydrotherapy represents an effective prophylaxis against common colds.