Scar development was investigated in 45 young patients who had undergone thoracic surgery. Patients were randomly assigned either to a group which was treated topically with Contractubex gel (Merz + Co., D-Frankfurt/Main), containing 10% onion extract, 50/U of sodium heparin per one g of gel and 1% allantoin, or to a group receiving no treatment. The treatment began on average 26 days after the operation and was continued for one year. The scars of all treated and untreated patients were evaluated at monthly intervals. The appearance of the scar, including scar type and scar size as well as scar colour, was assessed by the physician. A reduction of the increase of scar width was seen in the Contractubex-treated group as compared with the untreated group. Further, physiological scars and skin-coloured scars were more frequent in the treated group than in the untreated group. Hypertrophic or keloidal scars were less frequent in the treated group. No differences in scar length and scar height were seen. At the end of the observation period, the clinical course of scar development was rated as "very good" or "good" in more than 90% of the treated patients, "good" in less than 40% and "moderate" or "bad" in more than 60% of the untreated cases. The tolerability of the drug was "good" or "very good" in all cases. In conclusion, Contractubex gel is useful in scar treatment after thoracic surgery.