To investigate an abattoir outbreak of Q fever in southem New South Wales with reference to the protective effect and safety of the formalin-inactivated Q fever vaccine (Q Vax) administered before and during the outbreak. In September 1998, after notification of four Q fever cases in the abattoir, a cohort investigation of 103 workers was undertaken. Data on age, sex, immune status, vaccination status and main work area were obtained from the medical officer administering the vaccination program and abattoir records. Symptoms and occupational risk factors for illness were obtained from interview of 63 (61%) employees. Of 103 abattoir employees, 16 (16%) had immunity from previous Q fever exposure and 19 (18 %) had been vaccinated at least six weeks before the first case of Q fever exposure in the abattoir. Of the remaining 68 workers who were susceptible to primary infection, 29 (43%) had laboratory confirmed acute primary Q fever and eight were suspected cases. No workers vaccinated before the likely period of exposure developed Q fever. Of 32 workers vaccinated post-exposure, four developed laboratory-confirmed Q fever within eight days of vaccination. Vaccination administered 10 or more days after the likely period of exposure showed no significant protective effect (RR=0.57; 95% CI 0.13-2.57; p=0.60). Q-Vax was highly effective when administered in advance of the likely period of Q fever exposure. Post exposure vaccination was not shown to be protective. This study reinforces meat industry vaccination guidelines for abattoir employees. The optimal time to vaccinate workers is before they are put at occupational risk.