Significant differentials have been observed between Italian regions in the distribution of centenarians by sex, from two women per man in the south to over eight in certain regions in the north. In order to explain these differentials we studied the evolution of the femininity ratio (FR), using a longitudinal approach to follow the ageing process in two cohorts, and making use of nearly all the statistical data available since the time of the Unification of Italy in 1870. Significant differentials in the FR observed at the age of 100 are mainly due to mortality differentials among men over the age of 60. The high mortality of men in the north and their low mortality in the south are the main explanations of why the FRs are higher in the north and lower in the south. Mortality differentials among women have only a marginal impact.