The objective of the present study was to assess the sensitivity and feasibility of the Welfare Quality® (WQ) protocol for finishing pigs in 10 Spanish slaughterhouses. Sensitivity refers to the ability of the WQ assessment system to discriminate between slaughter conditions whilst feasibility denotes that the protocol is concise and easy to implement. On arrival at the plant, the incidence of dead, sick and panting animals was assessed in 1,002 (± 93) pigs per abattoir. During unloading, the percentage of pigs that slipped, fell, showed reluctance to move or turned back and were lame was also assessed. In the lairage pens, the stocking density and the percentage of pigs that were panting, shivering, and huddling was assessed in a total of 346 (± 81.0) pigs per abattoir. Stunning effectiveness, slaughter checks and skin lesions were also assessed in 60 animals per abattoir. For the majority of measures, any differences between slaughterhouses were found to be attributable to the installation itself and the management of the slaughterhouse, such as generalised fear, slipping and falling or stunning effectiveness, as opposed to measures taken to assess transport conditions or farm origin, such as lameness or sick and dead animals. The study protocol took 5.5 h for one observer to complete, in a slaughterhouse killing more than 550,000 pigs a year, although this time could increase dramatically in smaller abattoirs due to delays in the arrival of lorries. The protocol provides a general overview of the state of welfare of animals at the slaughterhouse and can readily identify specific problems in certain areas, such as stunning of animals.