The activity phase of BF Cygni discovered in May 1989 and which lasted about two years is examined. The UV emission lines, usually strong during quiescence, were not detectable during the outburst. UV high-resolution spectra in the outburst indicate the development of an expanding shell giving rise to a considerable number of violet-shifted absorption lines, especially from Fe II. Curves of growth analyses and radial velocity measurements of the Fe II lines indicate a decrease with time of the electron temperature and of the Fe II column density in the shell, while the expansion velocities increased. It is suggested that the activity phase of BF Cygni was triggered by an enhancement of the mass loss rate from the cool component taking place when the system was at periastron.