The probability of an AIDS patient being infected with tuberculosis (TB) has been studied in different populations and found to be increased by as much as 500 times, but the reverse, i.e. the probability of a patient with TB being infected with HIV, has not been studied. The aim of this study was to investigate the hypothesis of greater HIV seropositivity and altered immune status, as indicated by CD4+ T-lymphocyte counts, in TB patients. We prospectively studied 162 males, aged 18-30 years, hospitalized for active, proven TB. Serum for HIV antibodies was tested twice by ELISA and confirmed by the Western blot technique. The control group consisted of 145,000 blood donor volunteers serving in the army, aged 18-30 years. The number of CD4+ T lymphocytes was also measured in the patients and the control group. We found that the rate of HIV seropositivity in TB patients was 2.4% (4 of 162), while it was 0.214% in the control group (p < 0.0001). Using the Bayes’ theorem we found that the probability of a TB patient being infected with HIV was 9.1%, approximately 150 times higher than the expected rate in the matched control group (p < 0.0001). The number of CD4+ T lymphocytes was significantly lower in pulmonary and extrapulmonary TB patients than in the control group, taking into account the HIV status (p < 0.001). Our results suggest that there is a 150 times greater probability of a TB patient being infected with HIV. CD4+ T lymphocytes are significantly lower in all groups of TB patient