THE PLT STATED THAT BEFORE HE TOOK OFF FROM OKLAHOMA CITY, OK, HE CALLED THE OKLAHOMA CITY FSS & OBTAINED A WX BRIEFING. THE BRIEFING CALLED FOR LIGHT RAIN SHOWERS, MINIMUM CEILING OF 1800 FT, VISIBILITY OF 3 TO 5 MILES & NO CHANCE OF FOG, EXCEPT IN LOW LYING AREAS. HE AND ANOTHER PLT TOOK OFF IN A FLT OF 2 HELICOPTERS TO TRANSPORT A PATIENT FROM ARDMORE, OK TO OKLAHOMA CITY. AFTER ARRIVING AT ARDMORE, THE MISSION WAS CANCELLED WHEN THE PATIENT DIED. ABOUT 1 HR & 20 MIN AFTER ARRIVING AT ARDMORE, THEY DEPARTED FOR THE RETURN FLT AFTER RECEIVING ARDMORE ATIS. THE OTHER HELICOPTER WAS EQUIPPED WITH A RADAR ALTIMETER & LED THE WAY OVER MOUNTAINOUS TERRAIN. THE PLT OF N2005A WAS FOLLOWING ABOUT 3/4 MI BEHIND WHEN THE CEILING & VISIBILITY DETERIORATED TO ABOUT 500 FT & 1 MI. THE PLT SAID THAT HE HAD CLOSED ON THE LEAD HELICOPTER & WAS DESCENDNG & DECELERATING WHEN HIS ACFT COLLIDED WITH THE GROUND. IMPACT OCCURRED ON BANKED TERRAIN BESIDE A HIGHWAY.