Subsidized voluntary enrollment in government-run health insurance schemes is often proposed as a way of increasing coverage among informal sector workers and their families. This paper reports the results of a cluster randomized control trial in which 3,000 households in 20 communes in Vietnam were randomly assigned at baseline to a control group or one of three treatments: an information leaflet about Vietnam's governmentrun scheme and the benefits of health insurance; a voucher entitling eligible household members to 25 percent off their annual premium; and both. The four groups were balanced at baseline. In the control group, 6.3 percent (82/1296) of individuals were enrolled in This paper is a product of the Human Development and Public Services Team, Development Research Group. It is part of a larger effort by the World Bank to provide open access to its research and make a contribution to development policy discussions around the world. Policy Research Working Papers are also posted on the Web at http://econ.worldbank.org. The author may be contacted at awagstaff@worldbank.org. the endline, compared with 6.3 percent (79/1257), 7.2 percent (96/1327), and 7.0 percent (87/1245) in the information, subsidy, and combined intervention groups; the adjusted odds ratios were 0.94, 1.12, and 1.15, respectively. Only among those reporting poor health were any significant intervention effects found, and only for the combined intervention: an enrollment rate of 16.3 percent (33/202) compared with 8.3 percent (18/218) in the control group, and an adjusted odds ratio of 2.50. The results suggest limited opportunities to raise voluntary health insurance enrollment through information campaigns and subsidies, and that these interventions exacerbate adverse selection.