Figure 1. Seriologically confirmed cases of Ross River virus disase, by month of onset, in the southwest of Western Australia, July 1995 to February 1996, as reported by doctors to the Health Department of Western Australia (when possible, case follow-up questionnaires were administered by environmental health officers from relevant local authorities). Only a small number of cases diagnosed by state and private laboratories, although the patient was not notified, have been included. Consequently, the number of cases shown is almost certainly an underestimate of the true number of serologically confirmed cases. Almost 65% of cases have dates of onset in January 1996. However, further notifications and analysis of follow-up questionnaires that have not yet been carried out for many January/February cases may alter this pattern. Previous southwest outbreaks also peaked in January or February but were considerably less acute.