The effects of ZSM-5 addition in fluid catalytic cracking (FCC) are evaluated, Addition of ZSM-5 to the base catalyst in cracking of a hydrotreated flashed distillate at different residence times has as main result a higher production of LPG-range olefins. The results show that this is not solely caused by cracking of gasoline-range olefins by ZSM-5. ZSM-5 is able to produce LPG-range olefins (and gasoline) by cracking of feedstock-range hydrocarbons, probably after pre-cracking of a part or the paraffinic feedstock by the base catalyst or by thermal cracking. In the presence of the base catalyst, the activity of ZSM-5 for HCO cracking is comparable to the activity of the base catalyst and more than an order of magnitude larger than the activity of pure ZSM-5. Afterwards, when this pre-cracked feedstock has been consumed and significant amounts of gasoline-range olefins have become available, additional LPG-range olefins are produced by cracking of these gasoline-range olefins over the ZSM-5 additive. From a qualitative discussion of the results and a simple kinetic model it was concluded that the synergy is due to a combination of cracking of the feedstock molecules by the base catalyst and subsequently cracking of the olefins produced by the ZSM-5 catalyst.