  We combine the long-term optical light curve of the Seyfert 1 galaxy NGC5548
with the X-ray light curve measured by the Rossi X-ray Timing Explorer over 6
years, to determine the relationship between the optical and X-ray continua.
The X-ray light curve is strongly correlated with the optical light curve on
long (~year) time-scales. The amplitude of the long-term optical variability in
NGC5548 is larger than that of the X-ray variability (after accounting for the
host galaxy contribution), implying that X-ray reprocessing is not the main
source of the optical/X-ray correlation. The correlated X-ray and optical
variations in NGC5548 may be caused by instabilities in the inner part of the
accretion flow, where both the X-ray and optical emission regions may be
located.
