In the agroexport zone of Los Santos Zone in Costa Rica, coffee is harvested by migrant labor. Most migrants are from Panama and Nicaragua. We describe migrants' housing- and service-related health determinants, with analyses of ethnicity, nationality and geography. We used interviews, observation-based assessments, and the Geographic Information System to assess a population of 8,783 seasonal migrants and 1,099 temporary dwellings at a total of 520 farms during 2004-2005. We identified determinants of poor health including widespread deficiencies in the quality of grower-provided dwellings, geographical isolation, crowding, lack of radio and television, and deficient toilets and cooking facilities. The indigenous and non-Costa Ricans shared the poorest conditions. Reluctance to use mainstream public health services was widespread, especially among foreign and indigenous migrants and the geographically isolated. Post-study, researchers organized workshops for audiences including workers, coffee producers, public officials and service providers. Topics have included migration, preventive health and hygiene, and child labor. This work was successful in convincing Costa Rican social security authorities to implement reforms that improve access to and quality of health care for the migrants. Special projects on ergonomics, psychosocial health hazards, and water quality, as well as a literacy program, are ongoing.