In previous studies, the relaxant, anticholinergic (functional antagonism) antihistaminic, effects of Nigella sativa have been demonstrated on guinea-pig tracheal chains. In the present study, the prophylactic effect of boiled extract of N. sativa on asthmatic disease was examined. Twenty-nine asthmatic adults were randomly divided into control group (14 patients) and study group (15 patients), and they were studied for 3 months. In the study group 15 mL/kg of 0.1 g% boiled extract and in the control group a placebo solution was administrated daily throughout the study. Asthma symptom score, asthma severity, frequency of symptoms/week and wheezing were recorded in the beginning (first visit), 45 days after treatment (second visit), and at the end of the study (third visit). Pulmonary function tests (PFTs) were also measured, and the drug regimen of the patients was evaluated at three different visits. All asthma symptoms, frequency of asthma symptoms/week, chest wheezing, and PFT values in the study group significantly improved in the second and third visits compared with the first visit (P < 0.05 to P < 0.001). In addition, further improvement of chest wheezing and severity of disease on the third visit were observed compared with the second visit in this group (P < 0.05 for both cases). In the third visit all symptoms in the study group were significantly different from those of the control group (P < 0.01 to P < 0.001). However, in the control group, there were only small improvements in some parameters in just the second visit. The usage of inhaler and oral beta-agonists, oral corticosteroid, oral theophylline and even inhaler corticosteroid in the study group decreased at the end of the study while there were no obvious changes in usage of the drugs in control subjects. The results of phase I study generally suggest a prophylactic effect of N. sativa on asthma disease and warrant further research regarding this effect.