OBJECTIVE The public health significance of intestinal parasitism among migrant farmworkers has been poorly defined. We report a three-part study in which we attempted to provide population-based estimates and identify risk factors for infection. METHODS Parasite prevalence and incidence were estimated from two cross-sectional studies and a longitudinal study. We used questionnaires and clinical measurements such as hematocrits to examine risk factors and health effects. RESULTS In the 1987 convenience sample (n = 265), parasite prevalence ranged from 28% among the US born to 86% among the Central American born, with no significant difference between the prevalence of pathogens among US-born (14%) and Mexican-born (24%) subjects (P = .12, chi 2 test). High prevalences were also found in the 1988 random sample. An annual incidence of 9.5% (n = 74) was demonstrated in 1988. Symptoms such as abdominal pain were associated with infection, and lower hematocrits were associated with hookworm infection (P = .02, t test). CONCLUSIONS Infection with intestinal parasites appears to be an occupational hazard of farmwork, necessitating improved working and living conditions and greater clinical awareness.