After seven years since the activation of the Italian supplementary pension system, the workers voluntarily contributing to the new pension funds still represent a strict minority. This is in part due to the fact that not all the employees and employers have agree on instituting the appropriate funds. At the same time it must be recognized the relative youngness of the instruments. With respect to these traditional explanations, we introduced a different category of possible determinants, related to the unionized character of the instruments: the diffusion of the complementary pensions among the employees is mainly influenced by the presence of co-workers that are more sensible to the subject because unionized, i.e. being part of those who, by law, promoted the pension fund institution. With an adequate level of statistical significance, the empirical evidence supports the hypothesis and shows that the number of pension funds subscribers is strictly and directly related to the size of the firms. Due to the structure of the Italian economy this implies that the diffusion of the complementary pensions will in any case involve just a limited amount of employees. Most part of the workers and mainly those who are occupied in small firms will be de facto excluded from the access to a fundamental instrument devoted to the protection of their old-age incomes.