(1) In this paper, the relation between the tendency to leave a patch and the experience in that patch is studied, from a functional point of view. (2) The study is based on empirical data rather than the usual a priori modelling. The relative effects of several factors on patch-leaving decisions are estimated from the data and it is tested which factors have significant effects. Afterwards, we discuss the implications of the effects that were found in a functional context. (3) This approach is an important extension to a priori modelling, since it can indicate the relative importance of several aspects of the natural environment in determining the foraging strategy. (4) Experiments were done with the parasitic wasp Leptopilina heterotoma. In the trials, wasps were deposited in a Petri dish containing a patch of yeast with Drosophila larvae. Their behaviour was continuously recorded until they left the patch for more than 60 s. (5) The data were analysed by means of the proportional hazards model (Cox 1972) for factors affecting the leaving tendency of the parasitoids, i.e. the chance per unit of time that the patch was left. (6) It was found that the leaving tendency decreases with the number of ovipositions. (7) Oviposition rates experienced in the patch also have a strong influence on the tendency of the wasps to leave. As expected, the most recently experienced oviposition rates have the strongest effect. Long foraging times between recent ovipositions produce a high leaving tendency. (8) The number of times the patch has been left before, during periods shorter than 60