Utilisation of the semitendinosus and gracilis tendons in reconstruction of the anterior cruciate ligament (ACL) has become more common during the last few years. In recent studies a regeneration potential in the harvested tendons has been observed. In this study, 11 consecutive patients who underwent ACL reconstruction with a quadruple semitendinosus graft were examined 6-12 months postoperatively by MRI. Another two patients were examined within 2 weeks after surgery. The median age of the patients was 24 years and there were 8 males and 3 females. The right knee was involved in six patients and the left knee in five. A low-field 0.2 Tesla Siemens open MRI was used for examinations and T1 and T2 weighted transaxial sequences over the thigh and the knee joint were performed. In some instances, additional sagittal sequences were used. ROI analysis of the pixel value of the signal and area determinations on transaxial sequences was performed for both the involved and the healthy side. In 8 of the 11 patients examined 6-12 months postoperatively, a regeneration of the semitendinosus tendon with normal anatomical topographies to the level of the tibial plateau was found. Three of these eight patients were analysed more distally and fusion of the semitendinosus and gracilis tendons was found approximately 30 mm below the joint line before they inserted as a "conjoined tendon" into the pes anserinus. At the mid-thigh level, the semitendinosus muscle had a smaller area and a higher signal than that on the normal side. However, this difference was smaller in the patients showing normal distal tendon regeneration. This study indicates that the semitendinosus tendon has a strong potential for regeneration and that the muscle atrophy seems to be less in the patients with a more normalised distal insertion of the tendon in the pes anserinus.