BACKGROUND: Asthma and rhinosinusitis are common medical conditions among adults. Alternative treatments could have important impacts on health status among those individuals with these conditions, but specific prevalence data for these treatments are limited. OBJECTIVE: To estimate the prevalence of specific alternative treatment modalities, including herbal agents, ingestion of caffeinated beverages, homeopathy, acupuncture, and massage therapies. DESIGN: Random population telephone sample. SETTING: Northern California. PARTICIPANTS: Three hundred adults aged 18 to 50 years with self-report of a physician diagnosis of asthma (n = 125) or rhinosinusitis without concomitant asthma (n = 175). MEASUREMENTS: Structured telephone interviews covering demographics and clinical variables, including the following alternative treatments used in the previous 12 months: herbal agents; caffeine-containing products; homeopathy; acupuncture; aromatherapy; reflexology; and massage. RESULTS: Any alternative practice was reported by 127 subjects (42%; 95% confidence interval [CI], 36 to 48%). Of these, 33 subjects (26%; 95% CI, 21 to 31%) were not current prescription medication users. Herbal use was reported by 72 subjects (24%), caffeine treatment by 54 subjects (18%), and other alternative treatments by 66 subjects (22%). Taking into account demographic variables, subjects with asthma were more likely than those with rhinitis alone to report caffeine self-treatment for their condition (odds ratio, 2.5; 95% CI, 1.4 to 4.8%), but herbal use and other alternative treatments did not differ significantly by condition group. CONCLUSION: Alternative treatments are frequent among adults with asthma or rhinosinusitis and should be taken into account by health-care providers and public health and policy analysts.