Using a large sample of original and published observations of P Cygni, we found convincing evidence of a long-term cyclical variation in the H_alpha equivalent width and the UBV magnitudes and colors of the star. The data cover the interval from 1989 to 1999. The length of the cycle in the H_alpha EW variability is a few years and does not appear to be constant in time. The duration of the long-term photometric variation is about 7 years. In its colour behaviour, the star is redder when it brightens and vice versa, i.e. it resembles other LBVs during the so-called "normal S Dor phase". The long-term spectral variability appears not to be correlated with the variability in UBV.