We investigate the preservation of quality of service guarantees to a flow of packets in the presence of flow aggregation. In flow aggregation, multiple flows, known as the constituent flows, are merged together resulting in a single aggregate flow. Packet schedulers located after the network point where the aggregation occurred are aware of the aggregate flow, but are unaware of its constituent flows. In spite of this, we show that quality of service may still be guaranteed to the constituent flows if the aggregation is performed fairly. Furthermore, contrary to intuition, the quality of service guaranteed to a flow may be greater under flow aggregation than in the case where no aggregation is performed.