The need for long-term fiscal projections is self evident. Of these projections, pension expenditure is one of the most important since firstly it represents a large share of total expenditure, and secondly because of the positive correlation between this variable and demographic ageing. In this paper, we develop a model to project contributory pension expenditures in the Spanish Social Security System disaggregating the results by pension category, social security regime and sex. The most salient of the results obtained is the expected steady growth of total expenditure in contributory pensions. This would lie around 15% of GDP around 2045 compared to its initial level of barely 8% even though the baseline scenario incorporates a substantial recovery of employment and female participation rates. By pension categories, retirement pensions are those that determine the tendency of total expenditure evolution. Interesting conclusions can also be extracted from the analysis by sex. For instance, even accounting for an increase in female retirement pensions due to their higher participation, the corresponding increase in widow male pensions implies a higher total increase of the total number of contributory pensions accruing to men.