The paper describes an efficient and accurate numerical procedure for calculating the probability distribution function of combined first-order and slowly varying, second-order hydrodynamic loads and response of compliant offshore structures. No approximations are made except those inherent in the numerical calculations. The method does not require extensive computer capacity; in fact it can be implemented on any standard PC. Several example calculations serve to illustrate the method, and its accuracy is demonstrated by comparison with cases where exact analytical results are available. The accuracy of previously proposed approximations are also discussed.