In order to identify motorcycle accident cause factors and countermeasures in Thailand, a large prospective study was undertaken. Researchers conducted on-scene, in-depth investigation and reconstruction of 969 collisions involving 1082 motorcycle riders. Accidents were randomly sampled and included all levels of injury severity. Alcohol proved to be the most outstanding cause factor, with 393 drinking riders in crashes. Alcohol accidents were distinctly different from non-alcohol crashes. Alcohol accidents were more frequent on weekends and particularly at night, usually when the rider was on his way home. Drinking riders were more likely to lose control of the motorcycle, usually by running off the road. They were more likely to be in a single vehicle accident, to violate traffic control signals, and to be in non-intersection collisions. Males were far more likely to drink and ride than females. Drinking riders were far more likely to be inattentive to the driving task just before they crashed, and to be the primary or sole cause of the accident. One-fourth of all riders did not go to the hospital, and another 42% needed only treatment in the emergency room. Drinking riders were more likely to be hospitalized and far more likely to be killed. The higher hospitalization and fatality rates of drinking riders resulted from the kinds of accidents in which they were involved, not from the minimal differences in speeds and helmet use. Problems with balance and coordination were about equally rare among drinking and non-drinking riders. Inattention was a far greater contributing factor.