The heroin model used developed a consumption-based estimate of the amount of heroin that was produced in South America and Mexico. After accounting for seizures, U.S. consumption of South American heroin should approximately equal South American production. Likewise, U.S. consumption of Mexican heroin should be approximately equal to Mexican production. Only a small proportion of Asian heroin is consumed in the United States, so there is no practical way to equate U.S. consumption of Asian heroin to Southeast and Southwest heroin production. Based on this model, on a yearly basis Americans consumed about 12.3 metric tons of heroin between 1995 and 1998. Fifty percent came from Colombia, and 25 percent came from Mexico. Seizures accounted for 0.75 metric tons from Colombia, so Colombia would need to produce about 6.9 metric tons to satisfy the U.S. market. Only about 0.3 metric tons are seized from Mexico, so Mexico needed to produce about 3.4 metric tons to meet U.S. demand. The best estimate of the model was that approximately 12 to 13 metric tons of heroin were used in the United States during a given year; this level of use has not changed appreciably during the last several years. The fact that the consumption-based estimates were so close to the supply-based estimates was compelling but not convincing evidence that the heroin flow model provided an accurate profile of how much heroin entered the United States, how it got here, and from where it came. 5 figures, 8 tables, and appended analysis to determine seizures at U.S. entry points