Abstract Statistical analysis of data over a long period (1975–2005) shows that the magnitude and other characteristics of Forbush effects (for cosmic rays of 10 GV) are correlated with the power and longitude of the associated solar flares. These relations were obtained by using long-term observations of X-ray flares onboard GOES satellites, solar wind parameters and cosmic ray data. The averaged variations of galactic cosmic ray density were derived by the superposed epoch method for powerful flares that occurred within three different longitudinal intervals on the Sun. Their time profiles showed essential distinctions in both magnitude and shape. A magnitude distribution of the effects in cosmic rays and in Ap indices was obtained as a function of longitudes of solar flares associated with the interplanetary disturbances responsible for these effects. The East–West asymmetry was found for both the Forbush effects and magnetic storms: the large effects in cosmic rays are associated with the solar flares observed in the eastern or central regions of the solar disk, whereas the flares that occurred in the western portion of disk are more effective for the magnetic storms. The relation between solar wind maximum speed in the disturbance and mean transit speed of this disturbance was obtained with higher accuracy than before.