OBJECTIVE: Antiretroviral regimens containing the fusion inhibitor enfuvirtide (ENF) are associated with sustained viral suppression and immunological benefit. However, local injection site reactions (ISR) occur in the majority of patients. The aim of this study was to determine the pathogenesis of ISRs. METHODS: Injection sites were evaluated prospectively from 30 min up to 15-30 days post-injection in ENF-experienced (Cohort I) and ENF-naive patients (Cohort II) during the first 2 weeks of therapy. Four to five injections were given in rotating abdominal sites by a nurse using a standardized technique and were rigorously evaluated. RESULTS: Reactions were observed in 80-100% of patients; the majority of the reactions were mild to moderate, generally appeared within 24-48 h post-injection, and pain, induration and erythema were the most common clinical signs. Whereas most patients experienced ISRs, the overall prevalence in Cohort II was low (35% maximum). Punch biopsies of injection sites in Cohort I consisted primarily of mixed lymphocytic infiltrates with eosinophils and neutrophils. Injection vehicle (ENF buffer minus ENF) and reduced volume (2 x 0.5 ml ENF [45 mg] versus 1.0 ml [90 mg] ENF) were investigated in Cohort II. Fewer reactions appeared with vehicle and pain was absent with the smaller injection volume. Pathology was indistinguishable between ENF, vehicle and normal tissue in Cohort II patients. CONCLUSION: These results suggest that injection technique, injection volume and peptide may influence ISR to ENF.