On June 12, 2003, about 1140 Pacific daylight time, a Mooney M20J, N1156N, made a gear up landing at Lincoln Regional Airport (LHM), Lincoln, California. The borrowed airplane was being operated under the provisions of 14 CFR Part 91 as a personal flight. The private pilot, the sole occupant, was not injured; the airplane sustained substantial damage. The local flight departed LHM about 1045. Visual meteorological conditions prevailed, and a flight plan had not been filed. The primary wreckage was at 38 degrees 54 minutes north latitude and 121 degrees 21 minutes west longitude. The pilot reported in the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) that he failed to lower the landing gear because he was distracted by other aircraft in the pattern. He did not notice the gear up warning horn or that the airplane was handling differently. The pilot stated there were no mechanical failures.