In this study the hypotheses were tested that retention rates, days spent in methadone treatment and upward mobility to outpatient and inpatient treatment modalities for heroin-dependent people in a methadone programme are lower for clients from ethnic minorities as compared with the clients from the ethnic majority (Dutch). Anonymous data for 742 heroin-dependent people who entered methadone treatment in 1996 were extracted from a psychiatric case register. Their health service utilisation was analysed over a period of four years. Results show that ethnic minorities are under-represented in the methadone programme. They also spent fewer days in the methadone programme. No differences were found between ethnic minorities and the Dutch clients in their use of outpatient treatment. However, people of Moroccan and Turkish descent were under-represented in inpatient treatment. This is of concern because fewer days in treatment predict less than optimal treatment outcomes.