Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Flight Crew Private
Flight Crew Last 90 Days 32; Flight Crew Total 339.6; Flight Crew Type 195.7
Human-Machine Interface
Aircraft Equipment Problem Less Severe
Air Traffic Control Issued New Clearance; Aircraft Aircraft Damaged; Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport
Aircraft; Human Factors
Conducted pre-flight and pre-takeoff checklist. Item to check door latch is listed and verified twice and confirmed by passenger. Just after rotation; the pilots' entry door opened. Stabilized airspeed and climb; then notified tower of situation. Tower cleared left traffic to return to runway. Proceeded upwind; crosswind and downwind to set up approach to land. On downwind leg door failed and liberated from aircraft and was lost in the swamp/lake area south of the airport. Landed without incident. Called tower later to let them know the door was not lost on the airport operations area. Search for door on ground was unsuccessful.
Zenair pilot reports pilot entry door opening just after rotation.  On downwind during return to the airport the leg door departs the aircraft. A normal landing ensues.