A longitudinal study of leptospiral agglutinins in subjects five years of age and over was undertaken in Trinidad and Barbados between 1980 and 1982. Households were sampled randomly from one urban and two rural communities on each island, giving a total of 576 eligible individuals in Barbados and 524 in Trinidad. Participants were examined three times at approximately annual intervals. The prevalence of seropositivity at a titre of 1:50 using the microscopic agglutination test was 18.5% in Barbados and 21.9% in Trinidad. Prevalence increased steeply with age in both sexes and was higher in males than females on both islands. There was a marked difference in predominating serogroups on the two islands--Autumnalis (42% of positive cases) predominated in Barbados while Bataviae (29% of positive cases) predominated in Trinidad. Estimates of incidence rates for seroconversion were 2.9% per annum for Barbados and 3.5% per annum for Trinidad. Occupational risk varied between the islands, but in both cases highest seropositivity rates (greater than 50%) were found in outdoor labourers and lowest were found in indoor non-manual workers and urban homeworkers. In Barbados seroprevalence was higher among persons who cleared drains or who had contact with livestock. Lack of an inside toilet was associated with an increase in seropositivity on both islands. There was little evidence of household clustering of seropositive cases.