The laws regulating the financial markets allow the Swedish Financial Supervisory Authority (SFSA) and the courts to consider certain mitigating circumstances when deciding which sanctions to impose on an undertaking that has violated its obligations under the relevant laws. One such mitigating circumstance is cooperation with the SFSA during its investigation of the violation. Chapter 15 section 1 c of the Banking and Financing Business Act (2004:297), for example, states that the SFSA may consider whether the undertaking, to a significant extent, through active coop­eration, has facilitated the SFSA’s investigation. This paper explores two aspects of cooperation as a mitigating circumstance; first, by studying the law, relevant preparatory works, similar provisions in competition law and criminal law, and legal literature, what kind and extent of cooperation may be considered in decisions on sanctions, and second, by examining decisions by the SFSA and the courts regarding sanctions, how the law is applied in practice. The key finding is that such cooperation that significantly facilitates the SFSA’s ability to prove a violation should be considered relevant. This should arguably include cooperation making the SFSA aware of a committed violation – self-incrimination – as well as cooperation that takes place after the SFSA has initiated an investigation. It is also clear that only cooperation that goes beyond the expected level of cooperation is relevant. Cooperation with the SFSA has not been considered a factor in the decision on which sanction to impose in any of the cases decided during the investigated period. Due to a consistent lack of thorough explanations for the grounds of the decisions, no convincing conclusions may be drawn from the study regarding how the SFSA and the courts apply the law. However, cooperation with the SFSA is only one of several circumstances that should be considered when deciding on sanctions. It is therefore not unlikely, as some cases do indicate, that the SFSA and the courts in individual cases may disregard cooperation that would be considered sufficiently extensive, when other factors, such as the severity of the violation, support an overall view that it is not appropriate to consider mitigating circumstances in that particular case.