The chopper plunged into the Siutghiol Lake near the Black Sea this afternoon, carrying a pilot, a co-pilot, a doctor and a nurse.
Interior Ministry official Raed Arafat said the doctor was rescued and transported to a hospital in serious condition.
Divers and rescue workers were looking for the other three at the crash site, 250 kilometers east of Bucharest.
The search was hampered by frigid temperatures and darkness.
Arafat said the cause of the accident was not immediately known.