The viruses associated most frequently with the "common cold" are rhinoviruses and coronaviruses. The first prospective cohort study to determine the prevalence of rhinovirus and coronavirus infections in patients of all ages hospitalized for acute respiratory illnesses is described. Hospital admissions for acute respiratory illnesses were identified, and cell culture for rhinovirus and serologic assays on paired sera for coronaviruses 229E and OC43 were performed. A total of 61 infections with rhinoviruses and coronaviruses were identified from 1198 respiratory illnesses (5.1%); in addition, 9 additional infections associated with >/=1 other respiratory viruses were identified. Of those infected with only rhinovirus or coronavirus, underlying cardiopulmonary diseases were present in 35% of the patients aged <5 years, in 93% aged between 5 and 35 years, and in 73% aged >35 years. The predominant clinical syndromes varied by age: pneumonia and bronchiolitis in children aged <5 years; exacerbations of asthma in older children and young adults; and pneumonia and exacerbations of chronic obstructive pulmonary disease and congestive heart failure in older adults. Therefore, rhinovirus and coronavirus infections in hospitalized patients were associated with lower respiratory tract illnesses in all age groups.