The objective of the article is to explore the impact quality management systems and quality assurance activities in nursing homes have on clinical outcomes. The results are based on a cross-sectional study in 65 Dutch nursing homes. The management of the nursing homes as well as the residents (N= 12,377) participated in the study. Primary survey-data about the implementation of quality management systems and quality assurance activities were collected in 1994/1995 and in 1998, and were combined with information on resident characteristics and the prevalence of undesirable clinical outcomes. The results demonstrate that there are differences between nursing homes in the prevalence of undesirable clinical outcomes. In the nursing homes with the lowest scores, undesirable outcomes occur approximately 10 times less often than in nursing homes with the highest scores. The multi-level analysis has demonstrated that the differences in outcomes are mainly caused by differences between residents and, to some extent, also by differences between nursing homes. Resident characteristics explain 48% of the differences between residents and 72% of the differences between nursing homes. The size of the nursing home, the involvement of a client council and the implementation of a quality management system could explain a small part of the remaining variation in the number of undesirable outcomes. It seems that the implementation of a quality management system and the involvement of a client council had significant influence on the number of undesirable outcomes. Approximately 50% of the undesirable outcomes cannot be explained by the selected resident characteristics, the size of the nursing home and the implementation of quality management systems or quality assurance activities.