At sub-Antarctic Marion Island, there was substantial correlation in the numbers of adults breeding at study colonies of macaroni penguins (Eudyptes chrysolophus) over 26 years, as there was also for eastern rockhopper (E. chrysocome filholi) over 22 years, suggesting that overwintering conditions may influence the proportions of birds breeding. For both species the time of arrival of females for breeding, and for rockhopper penguins the mass of females on arrival, was significantly related to breeding success. Therefore, overwintering conditions may also affect breeding success. Trends in breeding success between study colonies were more strongly correlated for macaroni penguins than for rockhopper penguins. Macaroni penguins have a greater foraging range than rockhopper penguins when breeding, and may be more influenced at this stage by wider-scale environmental phenomena. For macaroni penguins, breeding success was significantly correlated with mass of chicks at fledging. For both species, mass on arrival of males was significantly correlated with that of females. Although both species had low masses on arrival after the El Niño Southern Oscillation event of 1997/98, there was no significant correlation in mass on arrival between the two species. It is likely that at Marion Island their overwintering grounds are different.