On July 5, 1997, at 1100 Pacific daylight time, a Cessna 170B, N37RA, registered to and operated by the pilot as a 14 CFR Part 91 personal flight, collided with trees shortly after takeoff from the Sunriver Airport, Bend, Oregon. Visual meteorological conditions prevailed at the time and no flight plan was filed. The airplane was substantially damaged. The commercial pilot was seriously injured and one of his two passengers received minor injuries, while the other one was not injured. The flight was destined for Klamath Falls, Oregon. The pilot reported in a written statement that during the preflight inspection on the airplane, he noted that the wind direction "was about 10 - 15 degrees off from runway 18." (wind direction from about 190 - 195 degrees). The pilot reported that he performed a run-up, magneto check and leaned the mixture prior to takeoff. The pilot reported that the takeoff ground roll "seemed somewhat long, but after using about 2/3 -3/4 of the runway, airspeed indicated 60 - 65 and we lifted off." The pilot stated that the climb out seemed normal until the airplane reached the end of the runway when it quit climbing and accelerating. The airplane was about 50 to 75 feet above ground level with tall trees directly in its path. The pilot stated that he turned the airplane to the south east (left) toward a meadow in hopes that the airplane would begin to climb. When the airplane would still not gain altitude, the pilot continued his flight path toward the golf course. The pilot stated that he had to pull up to clear power lines, however, after clearing the power lines, the pilot leveled the airplane and subsequently collided with trees. The nose of the airplane dropped, and the airplane collided with the terrain in a steep nose down attitude. The pilot stated that he felt that the wind direction or velocity changed toward the south end of the runway, as everything was normal up to that point. The pilot reported that there were no mechanical failures or malfunctions with the airplane at the time of the accident. Witnesses reported that the aircraft began its takeoff roll on the runway after the 1,000 foot displaced threshold. The airplane used most of the remaining runway before lifting off and attaining an altitude of no more than 50 feet, when it began to descend and struck trees approximately one mile off the south east end of the runway. The Sunriver airport is at an elevation of 4,159 feet. The runway is 5,500 feet long with a 1,000 foot displaced threshold. At the time of the accident, the temperature was 75 degrees, and the altimeter setting was about 29.99 "Hg. Density altitude was calculated at 6,118 feet. The engine was examined by a Federal Aviation Administration Aviation Safety Inspector from the Hillsboro, Oregon, Flight Standards District Office. The inspector reported that except for deposits of black soot on 11 of the 12 spark plugs, there was no evidence found of a mechanical failure or malfunction.