Four general frequencies of human St. Louis encephalitis (SLE) virus (epidemic, focal, sporadic, and no transmission) occurred in Florida between 1990 and 1999. An epidemic with 226 clinical cases and 11 deaths was reported from 28 Florida counties between July 1990 and January 1991. During the autumn of 1993, a focal outbreak was reported from Lee (5 cases) and Collier (3) Counties in southwest Florida. During the autumn of 1997, sporadic transmission to nine humans was reported from five Florida counties (Brevard [1 case], Polk , Charlotte , Lee , and Palm Beach ). Human infection with SLE virus depends on a number of variables that drive virus transmission. These include vector, virus, and avian host abundance, and meteorological events, especially rainfall. We monitored the abundance and serological status of wild avian amplification hosts, virus isolations from Culex nigripalpus Theobald females, and SLE virus transmission to sentinel chickens during 1990, 1993, and 1997. The epidemic of 1990 was characterized by conditions that produced an unusual abundance of vector mosquitoes and avian amplification hosts early in the year. We propose that epidemics may result when a specific combination of biotic and abiotic conditions favor SLE virus minimum field infection rates that approach 1:1,000 in Cx. nigripalpus vectors.