In music therapy, joint musical improvisation of therapist and subject provides the framework for a spontaneous and intimate, non-verbal interaction. This study shows that such an interaction can be used to reveal the subject’s capacity for emotional contact with another person, the nature of this contact and how well it is sustained. We examine the musical interaction in first music therapy sessions using a model of analysis specially developed for this study. A comparison between 15 schizophrenics, 15 depressed patients and 15 clinically normal controls revealed significant differences. The findings, which take into account subjects’ musical background and perceptual functioning, have implications for the diagnostic use of music therapy in adult psychiatry.