From all US Army enlistees leaving Vietnam in September 1971, a random sample of 943 men was selected. Of these, 470 represented a "general" sample of all enlistees returning at that time, and 495 represented a "drug positive" sample whose urine samples had been positive for opiates at the time of departure. We attempted to locate and personally interview all of the men in the samples. Results indicate that before arrival, hard drug use was largely casual, and less than 1% had ever been addicted to narcotics. In Vietnam, almost half of the general sample tried narcotics and 20% reported opiate addiction. After return, usage and addiction essentially decreased to pre-Vietnam levels. We discuss the use of nonnarcotic drugs, predictors and correlates of drug use in the samples, and the relationship of drugs to post-Vietnam social adjustment.