In order to assess the guidelines available in Dutch hospitals for the treatment of patients with serious infection of unknown aetiology, 39 antibiotic formularies used in 88 hospitals were analyzed. The recommendations considered were those for the treatment of sepsis for which the source was not apparent or which originated in the urinary tract, respiratory tract or abdomen. β-Lactam antibiotics (most commonly amoxycillin and cefuroxime) were the preferred agents for empirical therapy of infections of all types; an aminoglycoside was also included in the majority of regimens, irrespective of the clinical presentation. However, there were wide variations in the choice and dosages of the drugs administered. Because of the absence of local data for the susceptibilities of blood culture isolates, the appropriateness of the recommendations could not be properly evaluated.