Voice source parameters are often obtained by parametrizing glottal flow signals. However, before parametrization these glottal flow signals are usually low-pass filtered. As low-pass filtering changes the shape of the glottal pulses, it will also cause an error in the estimated voice source parameters. The present article presents results of our research on the effect of low-pass filtering on the estimated voice source parameters. We will first present an evaluation method which makes it possible to study the effect of low-pass filtering in detail. The evaluation results show that low-pass filtering leads to an error in all estimated voice source parameters.. However, the magnitude of the errors differs for the various voice source parameters, and also depends on the estimation method used. We will show that the errors can be reduced substantially by choosing the appropriate estimation method.