An analysis is presented of some of the data available concerning the epizootics of furunculosis that occurred in commercial fish farms in Norway and Scotland between 1984 and 1995. The primary aim of these analyses was to establish the plausibility of the hypothesis that the introduction of oil-adjuvanted vaccines (OAVs) was the major factor that resulted in the ending of these epizootics. The limitations of the epizootiological data available are recognised and, as a consequence, the analyses are limited to a comparison of the time course of these epizootics and the timing of the introduction of OAVs. With respect to the Scottish data, the decline in the furunculosis epizootic was shown to have preceded the introduction of the OAVs. In Norway, there was a closer temporal coincidence between the introduction of the OAVs and the decline in the epizootic. There were, however, grounds for suspecting that at least some decline in the epizootic in Norway occurred before the introduction of OAVs in that country. It is argued that the assumption that OAVs were the cause of the decline in the furunculosis epizootics is unwarranted. It is also suggested that the general acceptance of the existence of this causal link has inhibited investigation of other factors involved in both the rise and the fall of these epizootics. (C) 2000 Elsevier Science B.V. All rights reserved.