Patterns of fever, shock, and chills in 100 episodes of febrile, Gram-negative bacillemia were retrospectively analyzed to determine features predictive of the site of infection, organism, and prognosis. Pneumonias most often produced morning temperature rises, whereas infections in other sites were usually associated with an afternoon or evening peak. Peritonitis (usually due to Bacteroides fragilis) tended to cause an indolent temperature rise (over a day or more), whereas pyelonephritis and cholangitis typically produced an abrupt "spike." Relatively low fevers characterized Enterobacter pneumonias while very high fevers were noted in Pseudomonas aeruginosa infections in patients with leukemia. Chills occurred with unusually high frequency in cholangitis and in Klebsiella bacteremia. Patients going into shock had higher fevers than those who did not. More importantly, the development of shock was shown to be related to severity of underlying disease. Shock never developed if the disease was not serious, unless the bacteremia was caused by instrumentation, but occurred in 73% of patients with leukemia or lymphoma. The clinical setting, pattern of fever, and presence or absence of a chill can in many cases usefully guide diagnosis and therapy in patients with Gram-negative bacillemia.