A population-based register of cases of listeriosis admitted to acute-care hospitals has been established in Barcelona, Spain, in order to estimate the basal incidence of sporadic cases and to facilitate epidemiological surveillance of potential epidemics. Eleven acute-care hospitals reported all cases of listeriosis to a central unit following a standardized protocol. During 1990, 31 patients with listeriosis were identified, 18 of whom were residents of the city, resulting in an annual incidence of 10.95 cases per million inhabitants. Twelve of the 31 cases occurred in the period from July to September 1990, ten of them being community-acquired. The incidence of listeriosis was higher in elderly (> or = 65 years) and immunosuppressed persons. Forty-two percent of the cases were considered to be nosocomial infections. The overall mortality rate was 51.6%. The incidence of listeriosis in the present study is one of the highest reported in the literature. A high sensitivity of the reporting system with good case identification techniques, or demographic and environmental characteristics related to Listeria monocytogenes infection in our area, might be possible reasons for this geographic variation.