OBJECTIVE: To test the efficacy of steam inhalation in treating common cold symptoms. DESIGN: An in vitro study determined the temperature that inactivated rhinovirus: a temperature of 43 degrees C lasting at least 1 hour was needed. We then conducted a double-blind, placebo-controlled, randomized in vivo study. SETTING: The virology laboratory and the outpatient department of the Cleveland (Ohio) Clinic Foundation. PATIENTS: Sixty-eight Cleveland Clinic employee volunteers with symptoms of the common cold at the time of enrollment. INTERVENTION: A single 60-minute treatment was given to the volunteers. The steam treatment group (n = 32) received 40 L/min of heated saturated air that raised the intranasal temperature to 43 degrees C. The placebo group (n = 36) received 2 L/min of ambient air at 20 degrees C to 24 degrees C. MAIN OUTCOME MEASURES: Subjective symptom scores for nasal congestion, nasal drainage, and sneezing and objective measures of nasal resistance were studied during a 7-day follow-up observation period. RESULTS: There were no significant differences in daily symptom scores between the groups (P = .59 to .83). The only statistically significant differences between the groups were lower nasal resistances at baseline in the steam group (P = .04) and percent improvement in nasal resistance favoring the placebo group on day 7 (P = .01). However, these differences were of questionable clinical significance. CONCLUSION: We conclude that steam inhalation treatment had no beneficial effect on the cold symptoms of our volunteers.