Radiographic tibial and femoral bone tunnel enlargement has been demonstrated following anterior cruciate ligament (ACL) reconstruction. This study investigated whether bone tunnel enlargement differs between four-strand hamstring (HS) and patellar tendon (PT) ACL reconstructions over the course of a 2-year follow-up. Patients undergoing primary ACL reconstruction (n = 65) were randomised to receive either a PT or HS autograft. Femoral fixation in both groups was by means of an Endobutton. On the tibial side the PT grafts were fixed using a metallic interference screw, and the HS tendons by sutures tied to a fixation post. The PT grafts were inserted such that the proximal end of the distal bone block was within 10 mm of the tibial articular surface, resulting in a portion of free patellar tendon in the femoral tunnel immediately proximal to the articular surface. Patients were reviewed after 4 months and 1 and 2 years. Tunnel enlargement was determined by measuring the widths of the femoral and tibial tunnels with a digital caliper in both lateral and anteroposterior radiographs. Because of the presence of the interference screw and the proximity of the bone block to the tibial articular surface, the tibial tunnel could not be reliably measured in the PT group. Measurements were corrected for magnification, and changes in tunnel width were recorded relative to the diameters drilled at surgery. Standard clinical measures were also noted. In 32% of patients in the PT group there was femoral tunnel obliteration from 4 months onwards. For the other patients there was a significantly greater increase in femoral tunnel width in the HS group than in the PT group at each follow-up, but no significant change with time. There was also a marked increase in tibial tunnel width in the HS group at 4 months but not thereafter. There was no relationship between tunnel enlargement and clinical measurements. Although tunnel enlargement is more common and greater with HS grafts, it does not appear to affect the clinical outcome in the first 2 postoperative years. Femoral suspensory fixation does not in itself appear to be the principal cause of femoral tunnel enlargement, at least for PT grafts.