In this paper we describe a decision support system developed to help in assessing the need for various types of prison cells. In particular we predict the probability that a criminal has to be sent home because of a shortage of cells. The problem is modelled through a queuing network with blocking after service. The main objective of our study is to describe our analytical method and an approximate algorithm to solve this network. Through simulation studies we evaluate our method. Both the analytic and the simulation tool are elements of the decision support system. © 2000 Operational Research Society Ltd. All rights reserved.