Overall Ade´ lie penguin population size in Pointe Ge´ ologie Archipelago increased between 1984 and 2003 at a rate of 1.77% per year, and averaged 33,726±5,867 pairs. As predicted by the optimum model proposed by Smith et al. (Bioscience 49:393–404, 1999). Ade´ lie penguin population size increased when sea ice extent and concentration (SIE and SIC) decreased six years earlier, indicating that the conditions around reproduction or first years at sea, were determinant. The breeding success averaged 85.2±35.45% and was not related to environmental variables. Adult survival probability varied between years from 0.64 to 0.82. Southern oscillation index (SOI) had a strong negative effect on adult annual survival. Adult survival of Ade´ lie penguins increased during warmer events, especially during winter and spring at the beginning of reproduction. Therefore, we speculate that the rapid decreases in 1988–1991 and 1996 of the breeding population size were related to a decrease in adult mortality. However, adult survival varied little, and could not explain the strong increasing population trend. The sea ice conditions during breeding or during the first year at sea appeared determinant and influenced the population dynamics through cohort effects, probably related to the availability of productive feeding habitats.