The health care industry in some countries displays a gated structure. Rather than approaching a specialist directly, a patient will, in many cases, first seek a referral from a general practitioner. We provide one, but by no means the only, possible explanation for such an industry structure. If the outcome of treatment depends on the effort exerted by the treating specialist, and that effort is both unobservable and unverifiable, then an individual patient might be unable to motivate high effort treatment on the part of the specialist. By aggregating multiple patients, general practitioners can sometimes motivate high effort treatment by the specialist for patients when a single patient acting alone could not do so. If the cost of obtaining a referral does not exceed the additional benefit from high effort treatment, then a patient may, at least weakly, prefer to seek a referral before obtaining treatment from a specialist in such circumstances. (Journal of Economic Literature Classifications: C73, D82, I11.)