The General Accounting Office (GAO) reviewed available records of all aliens (2,705) who were detained at the time of its visits at 13 detention facilities to analyze INS detention policies and practices. INS can detain about 99,000 aliens a year at its current facilities, given the average 23 days of detention per alien in 1990. However, INS data show that about 489,000 aliens were subject to detention between 1988 and 1990 because they were criminal, deportable, or excludable. Due to a lack of detention space to hold them, INS has released criminal aliens and not pursued illegal aliens. GAO found that INS fails to treat excludable aliens consistently. Some aliens were released within a few days, and others remained in detention for extended periods of time. The detention of an alien and the length of time of the detention depended on the amount of available space where the alien eventually was detained, the location of the alien's apprehension, and laws and administration practices directed at certain nationalities. The INS need for increased detention space is symptomatic of larger enforcement issues related to aliens that remain unsolved and which require Congress and the Administration to determine whether and how to best control U.S. borders and remove aliens who are illegally in the country. 31 footnotes, 1 figure, 12 tables, and 3 appendixes