Research in maintenance engineering is increasingly focusing on complex systems in which components are subject to various dependencies, such as stochastic, structural, and economic dependences. This paper proposes a mathematical approach to calculate system’s availability with stochastic dependence and assesses its effect on the system. From this perspective, Cox Proportional Hazards Model (CPHM), based on the exponential survival distribution, is used. The results show that increasing the degree of dependence impacts, considerably, system availability.