A sample of 340 people was taken out of a group of 3500-workers who are building the San Carlos hydroelectric plant to find the allergic and parasitical indexes of the American tegumental leishmaniasis. AII of them had Montenegro's skin test and were screened for antibodies, using the indirect inmunofluorescence technique. The variable of person, time and place were recorded in a proper formulary. Biopsies were taken from 2 patients who had clinical lesions together with frotis with Wright's coloration and a culture in a NNN medium. An allergic index of 8.5%and null parasitical index were found. The inmunofluorescence was positive in 17 people or 5% of the sample. The titles were low (1:16) in most of them, and only two of them showed titles of 1:32. One of these had suspicious lesions but it was impossible to demonstrate the existence of the parasite. The study will be continued with 6 people who had a negative Montenegro's test and showed positive antibody titles for follow up. This study proved the existence of infection in the zone and it was concluded that it is possible that people get infected nowadays.