The photometric variability of 24 alpha Cyg variables, i.e. variable super-and hypergiants, observed by the Hipparcos satellite is described. Three of the selected stars are situated in the SMC, 12 in the LMC and the remaining 9 in the Galactic plane. Four of them are hot S Dor-type variables, or LBVs (luminous blue variables) and two are possible members of this class. Light curves are presented for all stars. For five stars, among which one LBV, the variability was discovered from the Hipparcos photometric data. Results of period searches are presented and, when relevant, folded light curves are shown. The linear ephemeris of two variables was revised. For most of the program stars the Hipparcos magnitude scale (Hp) differs from the V of the UBV system by la 0fm1 . For all variables temperatures and M_bol are given. Galactic foreground reddening for the objects in the Magellanic Clouds are given based on IRAS maps.