Within the framework of an interdisciplinary cooperation, the authors set up an on-site medical service provider in a specialized methadone substitution center in Germany. Here, they report on the prevalence of infectious and noninfectious diseases, and the vaccination status of substituted heroin-dependent patients.All patients who visited the medical care service provider between February 2008 and December 2008 were included in this study.Ten patients (7%) were seropositive for the hepatitis A virus. Two patients (1.3%) suffered from chronic hepatitis B; 40 patients (27%) were cured after a hepatitis B infection. Additionally, 99 patients (68%) were infected with hepatitis C virus (HCV), and 41 patients (28%) had active hepatitis C. Furthermore, 48 hepatitis C patients (33%) were cured. Of those, 25 patients (17%) cleared the virus spontaneously and 23 (16%) after ribavirin/interferon combination therapy. Ten (7%) of 146 patients were infected with the human immunodeficiency virus (HIV). Of those, four patients had active hepatitis C, and five patients were cured after a hepatitis C infection. 18 patients (12%) were vaccinated against hepatitis A and 28 (19%) against hepatitis B. Two of the 41 patients with chronic hepatitis C were vaccinated against hepatitis A. The most frequent noninfectious diagnoses were arterial hypertension (n = 28), bronchial asthma (n = 8), and diffuse liver parenchymal damage (n = 12).These results emphasize that i.v. drug users on substitution therapy are an underserved collective with a high prevalence of disease. The challenge consists in facilitating this population access to internistic and infectious disease service. The offer of an on-site medical service was well accepted. This is essential for an ongoing reduction of HIV and HCV prevalence in the drug users.