Neurosyphilis has been reported in HIV-infected patients previously treated with penicillin G benzathine, which does not achieve treponemicidal levels in cerebrospinal fluid. Therapy combining benzylpenicillin G and its repository form benzylpenicillin G benethamine could be a potentially effective alternative enhanced regimen for treating latent syphilis in HIV-infected patients because peak serum and cerebrospinal fluid concentrations would be achieved early post-administration by the former molecule and sustained for 24 h due to the prolonged half-life of the latter. In this study, 23 asymptomatic HIV and Treponema pallidum co-infected patients received 10 d of combined therapy (2 M IU intramuscular once daily) and were followed up at 3, 6 and 12 months. None experienced side effects or clinical symptoms. Of the 18 patients who were evaluated 1 y later, 8 (44.4%) exhibited serological treatment failure, defined as a positive serum rapid plasma reagin test. In conclusion, a 10-d regimen combining penicillin G and penicillin G benethamine seems to be of no benefit compared to currently recommended treatment.