This paper reviews the empirical literature on the relation between asthma, suggestion, and emotion, and proposes the hypothesis that these effects are mediated parasympathetically. The literature indicates that, among asthmatics, suggestion can produce both bronchoconstriction and bronchodilation, and that stress can produce bronchoconstriction. The proportion of asthmatic subjects showing bronchoconstriction to both suggestion and stress averages 35%-40% across studies, but, because of methodological considerations, might be conservatively estimated as closer to 20%. The effect is smaller for suggestion of bronchodilation, and is very short-lived among nonasthmatics. No clear connection has been found between these responses and such subject variables as age, gender, asthma severity, atopy, or method of pulmonary assessment, although some nonsignificant tendencies appear. Most studies in this literature used small n's and did not systematically examine various somatic, environmental, and demographic factors that could influence results. A hypothesis is presented regarding vagal mediation of psychological effects on the airways, as well as possible alternative mechanisms, and recommendations for future research to evaluate these hypotheses.