Purpose: This observational study was based on a series of clinical cases in which failure of sinus augmentations occurred in patients who received prophylactic clindamycin therapy. Materials and Methods: Between the years 2006 and 2010, a retrospective observational study was performed. The study consisted of 1,874 patients (723 males and 1,151 females) in whom sinus augmentations were performed prior to placement of dental implants. Results: In nine (0.48%) patients (four males and five females), infection of the graft material inside the sinus floor occurred, and six patients developed an abscess in the site of surgery, 4 to 6 weeks postoperatively. In three patients, a buccal fistula with pus draining was observed 5 to 8 weeks postoperatively. In all patients, the source of infection was from the grafted material within the sinus. A common manifestation in all nine patients was that they had self-reported penicillin allergy and had been prescribed clindamycin (300 mg every 6 hours for 10 days). Conclusion: Prophylactic clindamycin therapy following sinus augmentation procedures seems to be a risk factor for infections and loss of grafting material following these surgical techniques. Keywords: allergy, amoxicillin, clindamycin, infection, penicillin, resistance, sinus augmentation