In the literature, there are several different suggestions as to when the first postoperative pin-site care should be carried out to best prevent pin-site infections during the treatment by external fixation. In a cohort study, we compared the use of antibiotics and complications in patients where the theatre dressing was changed during the first postoperative week with patients where the theatre dressings were left undisturbed for the first postoperative week. Sterile compresses moistened with chlorhexidine 5 mg/ml in alcohol (70%), draped around each pin site and fixed by a bandage, were used as theatre dressing. In all patients, cultures were taken 1 week postoperatively from each pin site; use of antibiotics and complications during the treatment was documented. In 101 consecutive patients (118 knees) (73% men, mean age 50, mean BMI 27.5 kg/m2) operated on by high tibial osteotomy for knee deformity using the hemicallotasis technique, during 2005–2006, the theatre dressings were left undisturbed during the first postoperative week in 90 patients (104 knees) of group 1, and in 11 patients (14 knees) of group 2, the theatre dressings were changed during the first postoperative week. Eight of 11 patients in group 2 were treated with antibiotics compared to 32 of 90 patients in group 1 (RR 2.0, 95% CI 1.6–2.7, p = 0.02) during the treatment period by external fixation. Patients with a disturbed theatre dressing during the first postoperative week had an increased use of antibiotics by 18.6 days (95% CI 10.6–26.5, p < 0.0001, adjusted analysis). Four of 11 patients in group 2 had complications and 11 of 90 in group 1, adjusted analysis (RR 2.7, 95% CI 0.4–16.2, p = 0.3). Bilateral surgery simultaneously showed increased use of antibiotics by 10.4 days (4.4, 16.4, p = 0.0009) and increased risk of complications (RR 5.8, 95% CI 1.2–27.5, p = 0.03). In conclusion, the increased use of antibiotics indicates that leaving the theatre dressing undisturbed during the first postoperative week is beneficial to the treatment by external fixation and is probably of importance in the prophylactic pin-site care.