The aims of this study were to identify in what ways adolescents who cut themselves differ from those who take overdoses, and to investigate the role of contagion in these behaviours. Data from an anonymous self-report questionnaire survey of 6,020 adolescents in 41 schools were analysed. Comparison of 220 adolescents who reported self-cutting in the previous year with 86 who had taken overdoses in the previous year as the sole method of deliberate self-harm (DSH) showed that far more of those who cut themselves had friends who had also engaged in DSH in the same period (OR 2.84, 95% CI 1.5-5.3, P < 0.001), and fewer had sought help from friends before cutting (OR 0.5, 95% CI 0.3-0.9, P < 0.02). Self-cutting usually involved less premeditation. Analyses at both the individual and school level showed that the association between engaging in DSH and exposure to DSH amongst peers was largely confined to girls who cut themselves. There are important differences between adolescents who cut themselves and those who take overdoses. Contagion may be an important factor in DSH by adolescents, especially in girls who cut themselves. These findings are relevant to the design of prevention and treatment programmes.