The incidence of amyotrophic lateral sclerosis (ALS) ranges from 1.7 to 2.3 per 100,000 persons worldwide. Few epidemiological studies have been published in Latin America. The aim of this study was to estimate the incidence and prevalence of ALS in an HMO (Health Maintenance Organization) of Buenos Aires, capital city of Argentina. The population studied was affiliates of the Italian Hospital Medical Care Program, whose distribution across age and gender strata is similar to the population of Buenos Aires. Cases were detected from 1 January 2003 to 31 December 2010. Incidence density (ID) and prevalence for ALS were estimated for the whole period and at 31 December 2010, respectively. During the seven-year study period, the crude ID estimated was 3.17 per 100,000 person-years (95% CI 2.24-4.48) and the age-adjusted ID for the Buenos Aires population was 2.23 per 100,000 person-years (95% CI 1.45-3.01). Point prevalence at 31 December 2010 was 8.86 per 100,000 persons (95% CI 4.05-13.68). Mean age at diagnosis was 72.29 years (SD 8.5). In conclusion, estimated age-adjusted ID and prevalence of ALS were similar to the incidence and prevalence rates found in other geographical areas.