The present invention relates to a needle assembly having a build-in valve (30). The needle assembly is built up from a hub (20) carrying a needle cannula (10) and a telescopically movable shield (40) which axially covers the distal tip (13) of the needle cannula (10) between injections. The valve (30) is operated by the shield (40) and is arranged such that it is closed when the distal tip (13) of the needle cannula (10) is covered by the shield (40) and is opened when the telescopically movable shield (40) has been retracted a specific distance (X) proximally to the distal tip (13) of the distal cannula part (11).
