Pre-print, Working paper: This paper focuses on the switching behaviour of sickness fund enrolees in the Swiss health insurance system. Even though the new Federal Law on Social Health Insurance (LAMal) was implemented in 1996 to promote competition among health insurers in basic insurance, there still remains large premium variations within cantons. This indicates that competition has not been able so far to lead to a single price, and reveals some inertia among consumers who seem reluctant to switch to less expensive funds. We investigate one possible barrier to switching behaviour, namely the influence of the supplementary insurance. Our aim is to analyse two decisions (switching decision in basic insurance, subscription to supplementary insurance contracts). We use survey data on health plan choice and import some market data related to the sickness funds (number of enrollees, premiums). The decision to switch and the decision to subscribe to a supplementary contract are estimated both separately and jointly. The results suggest that holding a supplementary insurance contract substantially decreases the propensity to switch. However the impact of supplementary insurance is not significant when the individual assesses his/her health as "very good" ; to the contrary, holding a supplementary contract significantly reduces the propensity to switch when the indivual's subjective health status deteriorates. Futhermore, the switching decision is positively influenced by the expected gain of switching. In comparison with the range of the premium difference, the limitations to switch due to the supplementary insurance is moderate, though non negligible. As for the decision to subscribe a supplementary contract, the results show that the income level has a direct positive influence on the propensity to buy a supplementary insurance. Our results suggest that a major mechanism is going on in relation to supplementary insurance: holding a supplementary contract might stop individuals from switching when the individual thinks that she/he could be regarded as a bad risk due to the selection practices that are allowed in supplementary insurance markets. This result bears major policy implications concerning the regulation of basic and supplementary insurance markets.