We present a capital regulation policy in a model in which banks can choose to be unregulated, by operating in the shadow banking sector, when the cost of being regulated (restriction on portfolio risk) exceeds the benefit (cheaper funding/insurance). We show that the welfare maximizing capital requirement policy can be procyclical: lower requirement during booms and higher requirement during recessions. Our policy specifies the level of capital requirement as a function of the observed relative size of the unregulated and regulated banking sectors. This specification achieves the optimal aggregate risk exposure by obtaining the right mix of the two sectors.