HIV and syphilis have similar epidemiological characteristics which causes a high level of combined infection. Both STDs affect the central nervous system early after infection. Mental disorders occur with a high incidence in HIV-infected patients and patients with syphilis, but data on the effect of combined HIV and syphilis infection on mental disorders are found only in single articles. Objectives. The goal is to study mental disorders and their effect on the commitment to observation in the infectionist in HIV-infected patients with early syphilis. Materials and methods. A comparative study of 148 patients (65 HIV-infected patients with syphilis, 50 patients with syphilis monoinfection, 33 HIV-infected patients, seronegative for syphilis) was carried out. We used clinical, psychopathological, follow-up, psychometric, laboratory and statistical methods of investigation. Results. It was found that mental disorders occur in the majority (83%) of HIV-infected patients with syphilis with the predominance of affective (54%) and addictive (48%) disorders. In HIV-infected patients with early neurosyphilis, psychogenic reactions developed statistically significantly more often than in HIV-infected patients with early syphilis. In contrast, there were no statistically significant differences in the incidence of addictive, affective, personality disorders and mental disorders due to organic brain damage in patients with early neurosyphilis and early syphilis in the case of co-infection with HIV infection. Mental disorders due to organic brain damage had a mixed genesis (including infectious) in all cases and were associated and caused by the already existing HIV infection. HIV-infected patients with syphilis, in general, were characterized by the low commitment to observation in the infectionist which were influenced by the following factors: social maladjustment, drug abuse and criminal activity. Discussion. The frequency of detected mental disorders in the examined HIV-infected patients with syphilis (83,1%) corresponded to the literature on the incidence of mental illness in HIV-infected patients in St. Petersburg (85,6%), but was higher than the prevalence of mental disorders in patients with syphilis (68%). Given the established influence of neurosyphilis mono-infection on the development of mental disorders due to organic brain damage, it can be concluded that the disease with early neurosyphilis is important in the development of mental disorders of organic genesis. But the weight of this factor is insufficient in case of co-infection with HIV and early neurosyphilis and can only have additional significance in the development of mental disorders and the key factor is HIV-infection action. Mental disorders (addictive disorders and cognitive impairment) adversely affect the commitment to observation in the infectionist of HIV-infected patients with syphilis, therefore timely correction of mental disorders may be one of the factors improving compliance of patients. Conclusions. The study found a minor role of early neurosyphilis (as opposed to HIV infection) on the formation of mental disorders in the case of a combination of these infections. At the same time, it was found that addictive and cognitive symptomatic complexes have a negative impact on the probability of reference to an infectious disease specialist for initiating therapy in HIV-infected patients with syphilis.