Abstract Produced water from offshore installations is the largest discharge in the marine environment. The composition of produced water can be very complex and may vary in time, depending on for instance the maturity of the gas- or oilfield. Within the OSPAR region, i.e. the North-east Atlantic, now only the concentration of oil that is discharged is regulated. Therefore, it was decided within the OSPAR commission to develop a holistic risk-based approach for the assessment of produced water discharges and to incorporate Whole Effluent Assessment (WEA) within this approach as an instrument to fulfill the criteria of ‘holistic’. Therefore, in 2010 a practical program was performed on 10% of the offshore installations in the North-East Atlantic. Both an effect-based and a substance-based risk assessment were executed, respectively using a battery of different bioassays or the chemical analyses performed on the same samples. Both approaches were compared with each other. In this way, the added value of the WEA approach can be evaluated. Sampling and performance of the bioassays did not reveal any important practical constraints. Furthermore, large differences in toxicity exist between samples, indicating that WEA is sensitive enough to discriminate between samples. Besides, none of the bioassays could be designated as most sensitive. This emphasises the importance on establishing a risk assessment on more than one bioassay. It was shown that the susbstance-based risk assessment is in general less stringent than an effect-based risk assessment, and several reasons can be attributed to this differences. This Practical Progam has shown that WEA can be a valuable tool for assessing the environmental impact of produced water discharges. In the mean time, this effect-based assessment has been implemented within the Risk Based Approach in 2012.