This study set out to examine the impact of a health and personal development programme (the Social, Personal and Health Education Programme) which had been "enhanced" by the addition of a mental health promotion component. Students aged 12-16 years attending 17 secondary schools were randomly allocated as clusters to participate in either the standard programme (SP) or the enhanced programme (EP). Both programmes were delivered over an eight-month period. Questionnaires assessing help-seeking, emotional and behavioural difficulties and students' views of school social environment were completed by students in both groups at pre-intervention, post-intervention and at six-month follow-up. One thousand and seventy-two students participated. Analyses were carried out on the 782 students (72.9%) for whom data were available at all three time points. Both groups showed improvement over time in terms of emotional and behavioural difficulties, with male students identified as "at risk" in the EP showing significantly greater improvements than males "at risk" in the SP. Although there were few differences in outcomes between the groups, males identified as "at risk" appeared to benefit selectively from the EP.