Non-suicidal self-injury (NSSI) is a prevalent behavioral problem, yet many fundamental aspects of NSSI remain unknown. This case series study reports on the diagnostic correlates of adolescents with a recent history of NSSI and examines the relation between NSSI and suicide attempts. Data are from clinical interviews with 89 adolescents admitted to an adolescent psychiatric inpatient unit who engaged in NSSI in the previous 12 months. Results revealed that 87.6% of adolescents engaging in NSSI met criteria for a DSM-IV Axis I diagnosis (M=3.0, S.D.=2.2, range=0 to 8 diagnoses), including externalizing (62.9%), internalizing (51.7%), and substance use (59.6%) disorders. Most adolescents assessed also met criteria for an Axis II personality disorder (67.3%). Overall, 70% of adolescents engaging in NSSI reported a lifetime suicide attempt and 55% reported multiple attempts. Characteristics of NSSI associated with making suicide attempts included a longer history of NSSI, use of a greater number of methods, and absence of physical pain during NSSI. These findings demonstrate the diagnostic heterogeneity of adolescents engaging in NSSI, highlight the significant overlap between NSSI and suicide attempts, and provide a point of departure for future research aimed at elucidating the relations between non-suicidal and suicidal self-injury.