Although theory suggests that companies would rationally select into audit even if it were not a legal requirement, many countries impose mandatory audits. This is arguably due to an audit having elements of a public good, which may result in not enough audits being purchased without regulatory intervention. The mandatory nature of public company audit has created problems for researchers wishing to investigate the demand for voluntary audit. Recent events in the UK, however, have provided such an environment. In the UK, private companies must publicly file financial statements and, until recently, they had also to be audited. However, this requirement has now been relaxed for many private companies. We are therefore able to examine the determinants of voluntary audit in a large sample of companies for which we have financial statement data. We analyse a sample of 6274 recently exempt companies, following them for three years post-exemption. We use agency theory and prior evidence to generate our hypotheses and examine them using a more comprehensive set of explanatory variables than has previously been available in the literature. Our results indicate that companies are more likely to purchase voluntary audits if they have greater agency costs, are riskier, wish to raise capital, purchase non-audit services from their auditor, and exhibited greater demand for audit assurance in the mandatory audit regime. We also document a trend away from audit over time. Overall, our results strongly support the idea that companies choose to be audited when it is in their interests to do so.