An efficient Monte Carlo method for the calculation of radiative transfer in complex geometry systems including semi-transparent media has been achieved and validated. This method, which is based on the reciprocity principle and called optimized reciprocity method (ORM), can be applied to systems discretized in a large number of cells. For each pair of elementary cells exchanging radiative energy, the transfer calculation is carried out by minimizing, for a given computational time, the standard deviation of the radiative power or of the wall flux. The results obtained with ORM have been successfully compared to those obtained with previous approaches in a numerical benchmark. (c) 2005 Elsevier Ltd. All rights reserved.