The behavior of hepatitis A virus (HAV) infection among 980 members of 230 families in two rural districts of Costa Rica was studied prospectively from the recognition of the index case. The initial prevalence of detectable antibody (anti-HAV) ranged from 26.2% in children to 71.4% in adults. The ratio of index to household-associated infections was significantly higher among children than among adolescents and adults, indicating that children were most often responsible for the HAV introduction. The rates of household-associated cases among susceptible contacts were 70-83%; the final prevalences of anti-HAV were 90-95%. Neither index showed significant differences related to age. The ratio of clinical to silent infections in household-associated cases was uniformly 1.8:1 among children and adolescents; among adults, almost all associated infections were silent. Beginning with the 5-9-year age group, however, an immunoglobulin M response was absent in a progressively larger proportion of inapparent infections, strongly suggesting restimulation of specific immunoglobulin G antibodies by reinfection.