SummaryThe effect of tetracycline in the treatment of “Q” fever was investigated in 56 cases, by a technique of sequential analysis.Tetracycline reduced the duration of fever after commencement of treatment by approximately 50%. However, treatment must be commenced within the first three days of illness for this to be of importance.The severity of liver involvement, as estimated by “Bromsulphalein” retention, was significantly less in the treated group.The incidence of phase 1 complement‐fixing antibody six months or more after the onset was significantly less in the treated group.Treatment, essentially blind therapy, is probably justified only for patients who are severely ill, who are at risk of endocarditis because of previous cardiac disease, or who are in the older age groups.