Influenza B virus epidemics occurred in Houston, Texas, in 1976-1977 and 1979-1980. Among families with young children followed longitudinally in the Houston Family Study, 112 infections were detected during 511 person-years of observation. The infection rates for the two epidemics were similar--24 per cent and 20 per cent--although the two epidemics differed greatly in the community. The first epidemic was much more intense with a mid-winter peak that produced school absentee rates above 12 per cent for four consecutive weeks. The indolent epidemic of 1979-1980 smoldered from late September to mid-April with a peak during the second week of March for which school absenteeism did not exceed 8 per cent. In the Houston Family Study population, the combined infection rate for the two outbreaks was highest at 35 per 100 person-years for school children aged 6-19 years. Preschool children aged 7 months-5 years and adults had infection rates of 31 and 16 per 100 person-years, respectively. Preexisting neutralizing antibody titers greater than or equal to 3.5 log2 protected against influenza B infection and illness. Preschool children above 6 months of age, school age children, and parents introduced infection into the family at rates of 15, 15, and 9 per 100 person-years, respectively. Three second introductions were observed. The secondary infection rate was highest among school aged children at 61 per 100 persons at risk.