Similarly, the baseline characteristics were compared between the study populations in the six study sites involved in the study. Significant differences (Bonferroni correction; overall p-value <0.10) were found for ethnicity and on several substance use variables. The proportion of subjects with a non-Dutch ethnic background was particularly low in the site of Heerlen (2.1%), and relatively high in Rotterdam (26.1%) and The Hague (25.9%). The mean number of years of regular (i.e. at least three days a week) methadone use was significantly higher in the site of The Hague (14.8 years) than in the other sites (10.5-13.7 years). In addition, the duration of cocaine use was longest in the site of Amsterdam (10.7 years), compared to the sites in other cities (6.1-9.8 years). With respect to recent substance use, the number of days of heroin use in the month prior to the baseline interview was significantly lower in the site of Groningen (21.7 days) than in the other sites (25.2-27.7 days). Similarly, recent cocaine use was less frequent in the sites of The Hague (13.4 days) and Heerlen (12.3 days) than in the other sites (18.6-20.8 days).