Objective: Excessive internet use is being discussed as a non-substance-related addiction disorder. Estimations of its prevalence show that 1% of the German general population is affected by internet addiction. However, it is still unclear, whether internet addiction as a co-morbid disorder is also common among patients of the general health care system, especially in patients being treated in inpatient addiction rehabilitation centers. It seems plausible that these patients have a heightened proneness to comorbid internet addiction. Methods: In order to address this issue, a government-funded cooperation project was conducted. Within six months every new patient admitted to one of 15 rehabilitation facilities was screened for internet addiction. Results: 4.2% of the patients fulfilled criteria for internet addiction. Male patients of younger age with main diagnosis of cannabis dependence or pathological gambling were found to be at higher risk. Conclusions: Internet addiction is more common among patients of the rehabilitation system than in the general population. Especially young, male patients in treatment for cannabis dependence or pathological gambling are at risk for co-morbid internet addiction. It is recommended that regular screening for internet addiction should be implemented in facilities with patients at risk in order to offer indicative treatment.