The objective of this study is to describe the characteristics of the patients "lost to follow-up" and determining factors of lost to follow-up at the patients infected by HIV. This is a descriptive and analytical retrospective study made on patients with or not by HAART, registered in the Day hospital of Ouagadougou. Of 5118 adult patients studied, 402 (7.9%) lost to follow-up. Among these patients, 340 (84.5%) had an unknown vital status, 28 (7%) were alive and 34 (8.5%) died. Mean age was 37.5 years. After active research, 16 from 21 patients under HAART were in treatment interruption. The main factors associated with the loss of follow-up were: no schooling (p=0,008), residing outside the capital (p=0,002) and being infected with HIV2 (p< 10(-3)). The phenomenon of loss of follow-up is important and concerned mainly not informed patients.