Although hundreds of state and local organizations provide homelessness prevention assistance, the Government Accounting Office (GAO) could not determine the effectiveness of this assistance because few assistance providers have the resources available to collect the client follow-up data needed for evaluation. Homelessness prevention programs, included in amendments to the Stewart B. McKinney Homeless Assistance Act of 1987, provide short-term financial assistance or counseling to people threatened with eviction or mortgage foreclosure. Prevention assistance usually takes the form of one-time rent, mortgage, or utility payments. At least six federal programs provide funds in support of these efforts, but state and local service providers report that the demand for such help is so great that programs run out of funds or need to limit their assistance to individuals. Assistance providers also report that they believe prevention aid has kept people from becoming homeless, but few have evaluated what types of assistance are most effective and most are not collecting the type of information needed to make this determination. GAO believes that client follow-up data should be collected so that limited resources can be devoted to programs that prove most effective. Statistical data are included on two tables and two graphs. Descriptions of federal programs providing homelessness prevention assistance and a list of programs reviewed are appended. (FMW)