The aims of the study were to determine whether the participants of a practitioners' network changed their prescribing behaviour according to the guidelines discussed and to assess the implications for drug costs. Guidelines concerning the prescription of lipid-lowering drugs, antidiabetics and cardiovascular drugs were introduced in quality circles with about 90 participants in a practitioners' network. All statutory health care insurance physicians of Hesse served as comparison group. The analysis of the prescription data demonstrates that the network physicians attained their pre-determined quality goals but differed with respect to the cost implications for the drug target classes. Higher prescribing rates for recommended though more expensive drugs do not necessarily raise the drug costs of the indication group, which is partly due to the substitution of other drugs. The implications on treatment costs by implementing a guideline should be assessed for all sectors of the healthcare system. This kind of data was not available for this study. It is becoming increasingly relevant to know the cost implications of a guideline recommendation since (suspected) higher costs appear to be a barrier for guideline adherence.