We used an administrative dataset covering approximately 90% of all Germans to investigate the determinants of regional differences in the utilisation of ambulatory services in the year 2008. There are great regional differences in Germany, in GP, specialist and psychotherapist consultations. By means of a regression model taking account of the spatial dependencies of the error terms, we can explain a considerable part of the variation in terms of differences in demography, health status and socio-economic features. In addition, we made use of data on pollutants, the supply of services and the number of hospital cases as explanatory variables, which all have a significant influence on utilisation but contribute considerably less to explaining the differences. Overall, we are in a position to explain 29–40% of the regional differences in ambulatory case numbers at the level of the 413 counties and 55–70% at the level of the 16 German states (Länder) by observable differences. Copyright © 2013 John Wiley & Sons, Ltd.