Aims: Screening for concomitant drug consumption is necessary in opiate substitution therapy of opiate-dependent patients. Adulteration of samples is a common problem in this setting. A recently developed polyethylene glycol marker system allows reliable identification of urine samples. In this study, we aimed to compare the rates of drug detection in conventional and marker urine samples. Design: This cross-sectional evaluation was performed in an ambulatory opiate substitution program. We studied 55 opiate-dependent patients (32 men, 23 women). In all patients we compared the rates of drugs detected in the marker urine with the most recent conventional urine control. Additionally we assessed the rate of marker urine manipulation. Findings: In the conventional urine controls, opiates and benzodiazepines were found in 3.6 and 27%, respectively, whereas in the marker urine controls, these rates were 33 and 40%, respectively. Signs of urine manipulation were present in 35%. The rates of concomitant consumption and urine manipulation were higher among the patients without than among those with take-home substitution. Conclusions: With the marker urine, an unexpectedly high prevalence of concomitant consumption can be found. Marker urine testing has a significantly higher sensitivity for the detection of concomitant drug use.