Among 210 cases of imported malaria, officially notified in Lombardy from 1976 to 1980, we have selected 142 hospitalized patients and, about these, we got detailed informations on the stay in malaria endemic areas, on chemoprophylaxis and on their principal clinical data. We have found statistically significant correlations between P. falciparum infection and all the most frequent values in every class of epidemiological parameters considered, particularly with provenience from tropical African countries, touristic motivation, stay less than 30 days in winter season, onset of the illness within 20 days after the return in Italy, diagnostic lag shorter than 10 days after the onset and fever remission 2 or more days after starting specific treatment. There is also a tight correlation between these clinical parameters and the absence of individual chemoprophylaxis. The definition of these factors of risk is particularly useful to plan out some programs of prevention that should managed by regional Centers.