The dynamic behaviour of an actual production-inventory control system, designed for a diffusion department, is analysed by means of classical control theory. The actual control rule acts on the basis of norms for the production progress derived from the application of the line-of-balance technique to the production-inventory system. Attention is paid to the steady-state behaviour, the impulse response and the variance amplification of the controlled system. The cause of the poor dynamic performance observed after implementation of the system in practice is releaved. Especially the variance amplification can be decreased without a general loss of performance with respect to other criteria. An alternative control rule is derived from the application of design notions from classical control theory to the production-inventory problem. The new control rule shows an improved performance in some respects over the actual control rule. The case presented shows that some serious errors in production-inventory control design can be avoided through analysis by means of classical control theory.