Like many other countries, the Netherlands has a health insurance system that combines mandatory basic insurance with voluntary supplementary insurance. Both types of insurance are founded on different principles. Since basic and supplementary insurance are sold by the same health insurers, both markets may interact. This paper examines to what extent basic and supplementary insurance are linked to each other and whether these links generate spillover effects of supplementary on basic insurance. Our analysis is based on an investigation into supplementary health insurance contracts, underwriting procedures and annual surveys among 1,700-2,100 respondents over the period 2006-2009. We find that health insurers increasingly use a variety of strategies to enforce a joint purchase of basic and supplementary health insurance. Despite incentives for health insurers to use supplementary insurance as a tool for risk selection in basic insurance, we find limited evidence of supplementary insurance being used this way. Only a minority of health insurers uses health questionnaires when people apply for supplementary coverage. Nevertheless, we find that an increasing proportion of high-risk individuals believe that insurers would not be willing to offer them another supplementary insurance contract. We discuss several strategies to prevent or to counteract the observed negative spillover effects of supplementary insurance.