A needle assembly includes a transparent or translucent housing with a fluid inlet and outlet end, a flashback chamber, and a venting mechanism therebetween. The venting mechanism includes a blocking member to control the fluid flow in the venting mechanism so that it flows along the longest path through the vent. Substantially axially aligned inlet and outlet cannulas extend from the housing and communicate with the chamber. A sealable sleeve covers the external end of the outlet cannula. Relative volumes of the cannulas, the chamber, and the sleeve are selected to provide rapid reliable flashback indicative of venous entry with an internal vent positioned within the housing to divide the interior into first and second chambers, with the second chamber being adapted to maintain a negative pressure therein relative to the external environment so as to inhibit leakage of blood from the needle tip on withdrawal from the patient.