The Welfare Quality® project aims to develop a European on-farm welfare assessment standard for pigs, amongst other species. A prototype monitoring system was developed for sows and piglets using predominantly animal-based measures of behaviour, health and physiology to assess welfare. The prototype monitoring system was evaluated on a total of 82 farms in the UK and The Netherlands, encompassing a wide variety of farming systems. Preliminary analysis of the data indicates that the incidence of clinical welfare problems were low, with 1.2, 0.8, 1.1 and 0.1%, respectively of pregnant and lactating sows recorded as having substantial skin lesions, bursitis and vulval lesions, and extremely poor body condition. Some clinical problems were more prevalent in certain types of feeding system than others, particularly the severity of vulval lesions with electronic sow feeding (ESF) systems. Fear of humans, assessed by the extent of withdrawal behaviour from an unknown human, was low, and median scores were similar for sows housed in groups indoors, outdoors and in stalls. Some form of stereotyped behaviour was observed on almost 75% of farms visited, with sham chewing the most commonly recorded stereotyped behaviour. System design affected the practicality of some measures, particularly on outdoor farms. Full investigation of the complete data set will enable a refined, final, on-farm monitoring system to be developed and benchmark standards established.