Investigations of respiratory illnesses and infections in Tecumseh, Michigan, USA, were carried out in two phases, together covering 11 years. During the second phase, there were 5363 person-years of observation. Respiratory illness rates in both males and females peaked in the 1-2 year age group and fell thereafter. Adult females had more frequent illnesses than adult males; illnesses were less common in working women than in women not working outside the home. Isolation of viruses fell with increasing age; rhinoviruses were the most common isolate. Influenza infection rates, determined serologically, suggested relative sparing of young children from infection with type A (H1N1) and type B. Infection rates were highest in adult age groups for type A (H3N2). The isolation and serological infection rates were used to estimate the extent to which laboratory procedures underestimated the proportion of respiratory illnesses caused by each infectious agent; data from other studies were also used in this estimation. Severity of respiratory illnesses was assessed by the proportion of such illnesses that resulted in consultation of a physician. Rhinoviruses produced the greatest number of consultations. Overall, physician consultations were associated with 25.4% of respiratory illnesses.