Severe slugging is a well-known instability in multiphase flow through a pipeline-riser system that is characterized by sharp pressure changes and violent flow fluctuations. This can cause safety and operational problems in the production of oil and gas. Particularly for deepwater risers, which can be up to 3000 m long, the operating pressure in the pipeline and riser can be high (that is a few hundreds of bar). Most lab experiments carried out so far for severe slugging were conducted at atmospheric pressure. The present study contains new experiments at elevated pressure. Thereto a new large-scale pipeline riser system was established with 300 bar maximum pressure. To study the effect of the pressure at the riser top on severe slugging, experiments for the stability boundaries and for the amplitude and frequency of the slugging were carried out for pressures in the range of 0-50 barg. The results show that an increased backpressure gives a reduced region of severe slugging in the flow pattern map, whereas it also mitigates pressure fluctuations and decreases the slug frequency. Based on these experimental results, the existing prediction model for the stability of steady state operation and the transition to severe slugging was modified to incorporate the effect of the backpressure. (C) 2017 Elsevier Ltd. All rights reserved.