The paper provides a recursion model for the calculation of the probability density function (pdf) of intersymbol interference (ISI) which is caused by the combined effect of linear channel dispersion and of nonlinear distortion. The nonlinearity introduces statistical interdependencies between the interfering symbols and these dependencies are implicitly taken into account in a trellis-structured recursion rule. The results were verified by time consuming Monte Carlo simulation and show, e.g., that the nonlinear characteristic of a high power amplifier (HPA) reduces in some cases the error probability caused by the linear dispersion. Surprisingly, the ISI due to the nonlinear amplifier is increased in the case of offset modulation.