Excess morbidity was studied during influenza A epidemics (1968-69, 1972-73) among children in a large prepaid group practice program. Excess rates of hospitalization for influenza-related conditions, primarily pneumonia and bronchitis, ranged from 5 per 10,000 (95 per cent confidence limits (CL): 1 to 9) for non-high-risk children to 29 per 10,000 (95 per cent CL: 5 to 53) for children with high-risk conditions. The relative increases in hospitalization rates were greatest for 5-14 year old boys: 278 per cent and 104 per cent increases for high-risk and non-high-risk boys, respectively. The absolute increase was greatest for 0-4 year olds. The excess rate of ambulatory medical care contacts, 2.6 per 100 (95 per cent CL: -1.6 to 6.8 per 100) was not statistically significant. Excess hospitalization rates among 0-14 year olds during epidemics were three to five times larger than those for persons between 15 and 64 years of age but only one-fifth the rate of persons over age 65.