Many descriptive studies of the homeless have been conducted in the USA (Rossi, 1989) and, to a lesser extent, in Europe and other industrialized countries (Brousse et al., 2002: Marpsat and Firdion, 1998, 2000; Meltzer, 1994). Similar studies have also been carried out in Spain (Munoz et al., 2003; Vazquez, Munoz and Rodriguez, 1999), and they have found results that are generally comparable to those found in other countries. The homeless population is largely male, with an average age around 40, low educational levels, high unemployment rates, and significant mental and general health problems.Although the psychosocial profile of the homeless population has been investigated extensively, there are not many studies comparing the profile of homeless people to that of the general population and of vulnerable people (e.g. the poor) who are not homeless (Koegel and Burnam, 1988). One of the problems with that type of study is that the concept of vulnerability to homelessness is not easy to define. At first, research centreed almost exclusively on mental health disorders as risk factors (Fisher and Breaky, 1991). Further studies have identified a number of additional variables involved in the risk of becoming homeless such as, for example, the lack of robust family or social networks (Unger et al., 1998), the lack of schooling and job training (Brooks and Buckner, 1996; Stronge, 1993), employment difficulties (Epel, Bandura and Zimbardo, 1999), health problems (Ropers and Boyer, 1987), and stressful life events (Munoz et al., 1999). It is also well known that socio-economic variables, especially poverty and difficulties to gain access to the housing market, may influence the onset of homelessness.In order to determine which variables are most important and their exact role in homelessness, some researchers have begun to introduce control groups that are similar to the homeless group in some specific aspects. The majority of the studies focusing on those groups have analysed single women with and without children (Metraux and Culhane, 1999), the chronic mentally ill, people with substance-abuse disorders (Clark, Teague and Henry, 1999), and the elderly in high-risk situations (Keigher, Berman and Greenblatt, 1991). Although these populations provide an interesting contrast, they may differ from the homeless with respect to important key variables such as their socio-economic level.The main objective of the investigation was to compare two vulnerable groups (i.e., homeless people and users of services for the homeless) with available data (on socio-demographic, employment, educational and health variables) from a representative sample of the Spanish general population (Juarez, 1994). Comparisons of data from homeless groups and the general population are relatively rare. Furthermore, it is also necessary to assess the specificity of the findings by including other groups of disadvantaged people. Thus, as in other comparative studies of vulnerable populations, we used a control group of people who have a regular place to sleep but who use services for the homeless (i.e., soup kitchens and public baths). The second control group made up of vulnerable housed people was included as a means of analysing factors that, besides economic difficulties, may differentiate the homeless population.I. The samples1. The homeless group (HG)Sampling of the homeless group was conducted in Madrid, Spain, during four weeks in February and March 1997. Madrid is a city of three million people. The most reliable estimate of the number of homeless persons, based on field samples and service centre statistics, is around 3,000 people (Vazquez, Munoz and Rodriguez, 1999). The city's social services network providing shelter to the homeless consists of 10 public and private year-round shelters and 2 emergency shelters, open during the winter. At the time of the study, all of the 12 shelters were filled to capacity (1,086 beds). …