A seroepidemiological study of a household cohort, using both clinical observational and molecular criteria was conducted in a periurban area endemic for E. histolytica infection. This longitudinal study was undertaken to determine the risk of asymptomatic cyst carriers to develop invasive illness. Zymodeme patterns of strains isolated from these patients were correlated both with the clinical presentation of disease and with the serological response against the M-17 ameba antigen and further compared with that found in 16 proven cases of amebic liver abscess. From a total of 163 housewives screened, 39, (24%) were asymptomatic cyst carriers; 31 of them (index cases) and 114 members of their households remained in the study over an 8 month follow-up period to detect ameba infection and illness. Of the household members at risk, 46 (40%) became infected within 6 weeks. None of the index or secondary cases developed ameba-related symptoms and cyst excretion followed a chronic persistent, intermittent, or transient pattern over the period of the study. Amebas were recovered and zymodemes determined in 19 of 71 (27%) cyst carriers. Ameba shed from each of these 19 carriers exhibited nonpathogenic zymodeme 1, except for one index case where zymodeme 2 was recovered in one sampling, and returned to zymodeme 1 in subsequent samples. Of 48 of 71 cyst carriers studied, antibodies to crude E. histolytica antigen were detected by ELISA in 16 (31%); antibodies to the M-17 fusion protein were found in 8 (16%) by ELISA and in 2 (4%) by Western-Blot (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)