This paper reviews 18 surveys of mental health problems among homeless adolescents and reports on a pilot study of the same topic conducted in Amsterdam. Sampling methods and measures of mental health are discussed. The reported estimates of mental health problems vary greatly, very probably because of methodological differences. Despite the different methods used, there seems to be considerable research evidence to support a high prevalence of mental disorders among homeless adolescents. The results of the pilot study of 50 homeless adolescents in Amsterdam are consistent with the surveys reviewed. A highly structured interview was conducted at all four services sites for homeless adolescents in Amsterdam. Of the homeless adolescents interviewed, 78% had at least one lifetime DIS/DSM-III-R diagnosis, and 64% had at least one 1-month diagnosis.