Leptospirosis has received little attention in Portugal because it is a rare disease. However, regional disparities in the distribution of the disease and the increase in the number of cases identified through the mandatory reporting system in 1997 warrant a review of the epidemiology of leptospirosis in Portugal from 1991 to 1997. The results of the study therefore suggest that: 1. The incidence of leptospirosis in Portugal is likely higher than that estimated based on reports and hospital discharge data; 2. According to data from the hospital information system, there was an increase in the number of cases in 1996 and 1997. This observation is corroborated by the increase in the number of reported cases in 1997. 3. The incidence of leptospirosis is higher among men than among women and increases significantly around the age of 60. The burden of leptospirosis in Portugal is likely greater than routine data suggest. Our next objective is to study the most common modes of transmission of the disease and the serotypes associated with leptospirosis.