This paper characterises optimal short-run monetary policy in an economy with monopolistic competition, endogenous firm entry, a cash-in-advance constraint and preset wages. Firms must make profits to cover entry costs; thus the markup on goods prices is efficient. However, a distortion results from the absence of a markup on leisure. This distortion affects the investment margin due to the labour requirement in entry. In the absence of fiscal instruments such as labour income subsidies, the optimal monetary policy under sticky wages achieves higher welfare than under flexible wages. The policy maker uses the money supply instrument to raise the real wage - the cost of leisure - above its flexible-wage level, in response to expansionary shocks. This induces a rise in labour hours and more production of goods and new firms.