Five patients who developed a fever of undetermined origin during hospitalization are discussed. No infection was found in any of these patients. Occurrence of fever was temporally related to intramuscular (im) analgesic injections. In four cases damage of subcutaneous tissue was evident, and in two cases gallium scans showed uptake of gallium at injection sites. All five patients had received hydroxyzine either alone or in combination with other im analgesics. In addition, a retrospective chart survey of patients hospitalized for back pain who had received more than 30 im analgesic injections was carried out. Ten of 29 patients had fever lasting one to 15 days that could not be explained by the presence of obvious infection or inflammation or by the use of a nonanalgesic drug. In the chart survey it was found that febrile patients had been hospitalized significantly longer and had received more injections of all types and more injections of combinations of analgesics per patient than had afebrile patients. Thus, repeated im injections of analgesics may be associated with fever, with or without signs of tissue injury. Particular care should be taken in deciding whether im administration of hydroxyzine is necessary. Injection sites should be carefully monitored.