It has been suggested that the unequal coverage of different socio-economic groups by supplemental insurance could be a partial explanation for the inequality in access to health care in many countries. We analyse the situation in Belgium, a country with a very broad coverage in compulsory social health insurance and where supplemental insurance mainly refers to extra-billing in hospitals. We find that this institutional background is crucial for the explanation of the effects of supplemental insurance. We find no evidence of adverse selection in the coverage of supplemental health insurance, but strong effects of socio-economic background. A count model for hospital care shows that supplemental insurance has no significant effect on the number of spells, but a negative effect on the number of nights. This is in line with patterns of socio-economic stratification that have been well documented for Belgium. It is also in line with the regulation on extra-billing protecting patients in common rooms. For ambulatory care, we find a positive effect of supplemental insurance on visits to a dentist and on number of spells at a day centre but no effect on visits to a GP, on drugs consumption and on visits to a specialist.