The ADSS cost study methods involved a multistage data collection and verification process. Initial data were collected through the ADSS Phase I and Phase II surveys from 1996 to 1999. Phase I was a telephone interview with the facility directors of a national stratified probability sample of approximately 2,395 substance abuse treatment facilities. Phase II included site visits to a subsample of 280 of the Phase I facilities conducted between August 1997 and April 1999. This study is believed to be the first study of treatment costs with validated cost data from a nationally representative sample of substance abuse treatment facilities. The results of this study found that the mean cost of a nonmethadone outpatient admission was only a third of the mean cost of a nonhospital residential admission. In terms of cost per enrolled client day, the lower cost of nonmethadone outpatient care was even more dramatic -- only one seventh of the cost of a nonhospital residential day. The ADSS cost study found that the cost per enrolled client day in outpatient methadone care was only marginally higher than the cost per enrolled client day in nonmethadone outpatient care. One of the most important conclusions was the variability in unit costs within a type of care. Within a type of care, the cost per admission would be expected to exhibit only moderate variation, and cost per enrollment day would be expected to show little variation. In fact, both variations proved to be relatively large. The smallest element of cost was the cost per enrollment day. The coefficients of variation of the means for nonhospital residential outpatient methadone, and nonmethadone outpatient were 11 percent, 9 percent, and 8 percent, respectively. Although comparison with other studies would be of interest, the estimates from the ADSS cost study are not directly comparable with those from similar studies because of differences in methods, variables, and national representation. 6 figures, 10 tables, 6 references