The role of regional primary-care organizations (PCOs) in health-care systems is not well understood. This is the first study to attempt to isolate the effect of regional PCOs on primary-care performance. We examine Divisions of General Practice in Australia, which were established in 1992. A unique Division-level panel data set is used to examine the effect of Divisions, and their activities, on various aspects of primary-care performance. Dynamic panel estimation is used to account for state dependence and the endogeneity of Divisions' activities. The results show that Divisions were more likely to have influenced general practice infrastructure than clinical performance in diabetes, asthma and cervical screening. The effect of specific Division activities, such as providing support for practice nurses and IT support, was not directly related to changes in the level of general practice performance. Specific support in the areas of diabetes and asthma was associated with general practice performance, but this was due to reverse causality and the effect of unobservable factors, rather than the direct effect of Divisions.