Computer simulations that are based on classical Monte Carlo methods are often used to assess the performance of digital communications systems. Unfortunately, in many practical cases, ordinary Monte Carlo simulation may require a prohibitive amount of computing time. In this paper, we present an exact analysis method for evaluating the performance of direct-detection optical code division multiple access (CDMA) transmission systems. The proposed method is featured as a quick and flexible Monte Carlo technique which substantially reduces the required number of conventional Monte Carlo simulation trials. In particular, it is demonstrated that the proposed method can estimate the performance of optical CDMA systems with a high degree of accuracy to warrant its use as a powerful tool for performance analysis, design, and optimisation of such systems.