This study investigates the level of knowledge about mechanisms of HIV transmission and risk behaviour for HIV infection in Turkish immigrants in Basel, Switzerland. In addition, the effectiveness of physician based HIV counseling in a general internal medicine outpatient clinic was evaluated. Two consecutive samples of 150 and 98 Turkish patients with a first clinic contact, were recruited 6 months apart. The first group was exposed to an interpreter assisted counseling on HIV prevention (intervention group), the control group received no systematic counseling. Knowledge about mechanisms of HIV transmission and risk behaviour for HIV infection was assessed by a 29 item questionnaire at baseline and by interview at follow-up. One year follow-up was possible in 49% of the patients. At baseline, Turkish patients had statistically significant lower global scores on knowledge about HIV than a second control group of 148 Swiss patients. Mean percentage scores of correct answers in the whole Turkish study population improved from 49.3% to 60.0% (p < 0.0001). However, the difference of gained knowledge between intervention and control groups was only of borderline significance (p = 0.059). Study design and low follow-up limit conclusions from this study. From 1992 to 1994 knowledge about HIV infection had improved in Turkish patients, but was still inferior to the knowledge of Swiss patients.