The narrow OIII lambdalambda4959, 5007 emission-line fluxes in the spectrum of the well-studied Seyfert 1 galaxy NGC 5548 are shown to vary with time. From this we show that the narrow-line-emitting region has a radius of only 1-3 pc and is denser (n sub(e) ~ 10 super(5) cm super(-3)) than previously supposed. The OIII line width is consistent with virial motions at this radius given previous determinations of the black hole mass. Since the OIII emission-line flux is usually assumed to be constant and is therefore used to calibrate spectroscopic monitoring data, the variability has ramifications for the long-term secular variations of continuum and emission-line fluxes, though it has no effect on shorter-term reverberation studies. We present corrected optical continuum and broad H beta emission-line light curves for the period 1988-2008.