This study evaluated the relative effects of horizontal and vertical misaim of low-beam headlamps. The approach involved analyzing light-output matrices of 150 production low beams, manufactured for sale in the U.S., Europe, and Japan. The specific analysis involved computing, for 225 locations in the central part of each beam pattern, the ratios of nominal intensity and intensity for vertical and horizontal misaim of up to 1.5°. The ratios greater than 1 log unit were considered to be of practical significance in terms of changes in visual performance and discomfort glare, and those greater than 0.5 log unit of likely significance. Only changes relative to visual performance and glare under nominal aim were considered; absolute levels were not examined. Furthermore, simultaneous horizontal and vertical misaims were not considered.The results indicate that horizontal misaim of the U.S., European, and Japanese lamps of up to 1.5° in either direction is unlikely to be of practical significance. For the U.S. lamps, vertical misaim is of practical significance when misaim reaches 1.5°, and is of likely significance at misaim of 1°. For the European lamps and the Japanese lamps of the European type, vertical misaim of 1° is of practical significance, and misaim of 0.5° is of likely significance. For the Japanese lamps of the U.S. type, vertical misaim of even 1.5° is unlikely to be of practical significance; likely significance is reached at 0.5° of misaim.The present findings imply that photometric standards for low beams should include less strict tolerance limits for horizontal aim than for vertical aim.