The light curve of the outburst of Comet P/Metcalf-Brewington (1991 I) is investigated. The parameters of the light curve are quite uncertain as it may be fitted both in terms of variation with heliocentric distance and exponential decay with time however, after maximum a empirical fit of m1 = 4.1 + 5 log Delta + 14.8 log r gives a good fit to the data. The absolute magnitude brightened at least 11 magnitudes in outburst. The outburst is most plausibly caused by the release or micron sized grains as, were the outburst due to dust emission, it would imply the expulsion of a large fraction of the total mass of the nucleus.