A longitudinal study of trachoma was conducted among 100 members of nine families living in a hyperendemic area of Tanzania. Family members were examined for trachoma every three months for one year and conjunctival specimens were collected for antigen detection, which was performed either by direct fluorescent antibody cytologic analysis or enzyme immunoassay. The serovar specificity of tear antibodies was determined. Overall, young children tended to form a core of those with persistent, often severe, disease who consistently shed Chlamydia. These children are a potential source of infection in their family. Chlamydia could be identified early in the course of presumed recently acquired infections, but not later in resolving infections. This temporal change may account for the discrepancies between demonstrable organisms and clinical disease seen in cross-sectional studies. Several children were identified who did not develop trachoma despite having Chlamydia identified in conjunctival scrapings. Their ability to resist infection may offer clues for vaccine development. The study of serovar specificity is consistent with the intrafamily transmission of trachoma, but was confounded by the large family size and the potential for separate transmission units to occur within large extended families. These observations give further understanding of the natural history and kinetics of the transmission of trachoma that should be of use in developing and evaluating intervention studies.