Iceland's 2011 draft constitution is reputed to be the world's first crowdsourced constitution. Careful study of this case can therefore inform our understanding of how public participation in lawmaking may be supported by new technology. This article seeks to measure and explain the impact that comments and proposals from the Icelandic public had on the development of the draft constitution. The analysis shows that almost 10 percent of the proposals submitted by the public generated a change in the draft text, with public participation having a particular impact in the area of rights. This remarkably high impact is likely explained by the unique, apolitical context in which the constitution was drafted. Although on the face of it an example of successful public crowdsourcing, the new constitution drafted through this process ultimately failed to be ratified by the parliament. Iceland's experiment with participatory constitution drafting thus demonstrates both the possibility of impactful online public engagement, and the potential pitfalls of such a politically disconnected process.