This paper describes a numerical model of the dilution and disappearance of sewage discharged to the coastal zone. The model is based on the Monte Carlo (or random walk) principle. A cloud of particles is released at discrete time steps and the 3-dimensional path of every particle is simulated. The stochastic element of the movement is controlled by random numbers. It is shown how the model can simulate the unsteady case of the dilution from a sea outfall where both wind-induced and tidal currents are taken into consideration. The intention of the paper is to present how complex transport/dispersion phenomena can easily be modelled by the stochastic approach without going into advanced methods as finite differences or elements. The advantage of this approach is the simple programming and Iow need of computer memory. The disadvantage could be the need for excessive computing time.