Advisory services for farmers are supposed to contribute to the reduction of nitrogen emissions. There is not enough evidence on their effectiveness and cost-effectiveness. As a case study, an impact analysis of a Hessian water protection advisory service was carried out with a set of indicators based on empirical and theoretical principles.The written standardized survey with responses from 1,477 advised and non-advised farmers was supplemented by qualitative methods. As a result of the advice, reductions of N-surpluses between 2 and 5.5 kg N/ha per year were estimated, resulting in a cost-effectiveness of 8 to 26 € per kg of avoided nitrogen emissions - after only a very short period of advice of about 2 years. The costs must be considered in the context of synergies with other objectives, such as soil protection and biodiversity. Derived from the case study results and literature, success and comparison factors for advisory services as well as for their use in a policy mix are presented.