With the aim of knowing if a relationship exists between cost inefficiency and instability of financial entities before the 2008 financial crisis, a stochastic frontier approach is applied to Spanish savings banks in the period (2002-2007). Moreover, translog financial and operative cost functions are estimated by Maximum-Likelihood to determine the influence of each field of cost efficiency on financial instability. The results show that biggest inefficiencies are associated with operative cost, so management policies should be specially oriented to reduction of physical capital. On the other hand, the financial covariates show that the most effective strategies in this field consist of looking for advantages in deposits market and increases in resources and in quality of assets through provisions.