The product life cycle is a well known concept in marketing theory. It is generally assumed to comprise four phases. So far, however, quantitative evidence of this shape of the product life cycle has been missing. This paper deals with methods of estimating the parameters of such life cycles and two methods of estimating product life cycles are given. An iterative method is developed that is of independent value as well as providing a good starting point for Marquardt's generalized least squares method. It is shown that there exist complementary and substitutive effects between the product under consideration and the sales of related products sold by the same company.