Purpose: To assess the presence of HNP1-3 in the gingival crevicular fluid (GCF) of patients suffering from aggressive periodontitis before and after nonsurgical periodontal therapy. Materials and Methods: Twenty patients, each with generalised aggressive periodontitis (GAP) were included in the study. After periodontal examination, one site with a probing depth (PD) ≥ 4 mm was selected. Patients received nonsurgical treatment (scaling and root planing [SRP]) with additional administration of systemic antibiotic therapy (amoxicillin 375 mg three times daily + metronidazole 250 mg three times daily for 7 days). Prior to therapy and 3 and 6 months after, the following parameters were evaluated from the same site: PD, gingival recession (GR), clinical attachment level (CAL), plaque index (PI), bleeding on probing (BOP), sulcus fluid flow rate (SFFR). The level of HNP1-3 in GCF was determined by means of a commercially available ELISA kit. Results: Compared to baseline, the level of HNP 1-3 did not show statistically significant differences at 3 and 6 months. The evaluated clinical parameters and SFFR showed statistically significant decreases compared to baseline. At 6 months, PD (median) decreased from 7 to 3.5 and CAL (median) decreased from 7 to 4. Conclusion: In patients with GAP, nonsurgical periodontal therapy in conjunction with systemic administration of amoxicillin and metronidazole had no effect on the level of HNP1-3 in GCF. Keywords: aggressive periodontitis, α-defensins, gingival crevicular fluid, HNP1-3