The efficacy of Multisystemic therapy (MST) in treating adolescent aggression has been established, however, not all youth and their families benefit from MST. One reason for this treatment variability could be the failure to distinguish between different aggressive subtypes with different risk factors, developmental prognoses and treatment needs. We investigated whether changes in maternal depression over MST would lead to different outcomes for two aggressive subtypes: pure externalizers (EXT) and mixed externalizers/internalizers (MIXED). Forty-two EXT and 27 MIXED youth and their families underwent MST for six months. Maternal depression, youth externalizing and internalizing behaviour were assessed before and after MST. Results showed a marginally greater change in externalizing for EXT youth. In addition, reductions in maternal depression were related to successful treatment outcomes for MIXED youth only. Our findings have implications for MST clinicians, namely the importance of reducing depressive symptoms in mothers of MIXED youth to improve their outcomes. (Contains 2 tables and 2 figures.)