The effects of steam inhalation on nasal patency and on nasal symptoms were studied in 62 patients with the common cold by a double-blind, randomized, placebo-controlled clinical trial. Treatment consisted of two 20-minute sessions, during which the patient inhaled saturated, hot (42 degrees to 44 degrees C) air through the nose. The subjective response was recorded by each patient during the week following treatment on a daily symptom score card. Nasal patency was determined before treatment, the following day, and 1 week later by measuring peak nasal expiratory and inspiratory air flow. Highly reproducible results were obtained by using these objective methods. Steam inhalation resulted in alleviation of cold symptoms and increased nasal patency in a significantly higher percentage of patients in the actively treated group than in the placebo-treated group. Possible explanations for the effectiveness of treatment are discussed.