Guided bone regeneration (GBR) is a reconstructive procedure for treating atrophic alveolar ridges. This study aims to assess the correlation between different glycemic control levels and clinical findings in patients undergoing horizontal GBR before implant placement. The study population consisted of all patients requiring horizontal GBR. Patients were divided into three groups based on HbA1c levels: non-diabetic normoglycemic patients (HbA1c < 5.7%), non-diabetic hyperglycaemic patients (HbA1c < 6.5%), and patients with controlled diabetes (HbA1c < 7%). The primary outcomes were the horizontal (mm) and vertical (mm) dimensional changes of the alveolar ridge 6 months after the procedure. The study sample consisted of 54 patients. Sixty-eight implants (95.8%) were classified as “successful,” meaning the possibility of inserting a standard-sized implant following the GBR (diameter ≥ 4 mm). There was a statistically significant difference between the three groups in terms of horizontal gain at 6 months: in particular, there was a statistically significant difference between group 1 and group 2 (p= 0.026) and between group 1 and group 3 (p= 0.030). The present investigation showed that patients with HbA1c levels below 7% could undergo GBR and obtain a statistically significant horizontal bone gain.