The serum magnesium concentrations are compared between the three clinical classification categories established in 1993 by the Centers for Disease Control (CDC) (infection) by the human immunodeficiency virus) in adults, and its relation with the CD4 lymphocyte count, albumin, and pre-albumin. The serum magnesium concentration is part of the broad analytical screening and it is determined by atomic absorption spectroscopy. The data obtained were analyzed by the statistical program SPSS. The comparisons of the serum magnesium concentrations between the three clinical categories of the CDC were evaluated by an analysis of variance (ANOVA test). Afterwards, and using a regression analysis, the relations between the different variables were evaluated. 35 patients affected by HIV-AIDS were studied, 22 belonging to category A, 3 to category B, and 10 to category C, 29.4% of the patients studied presented hypomagnesemia, with an average serum magnesium level of 17.6 +/- 10.3 mg/l. There is a quadratic relation between the serum magnesium level and the number of CD4 lymphocytes in HIV-AIDS patients studied. A larger number of cases would allow us to conclude whether hypomagnesemia is a factor belonging to the disease, as well showing the possible differences in the magnesium state between the three diagnostic groups studied. Future studies will be necessary to clarify the role of magnesium in the clinical progression of patients infected by HIV.