BACKGROUND: The usefulness of the tuberculin skin test (TST) for HIV-infected subjects may be questioned because false-negative results are increasingly common with advanced immune deficiency. The objectives of this research were thus to describe the use and the results of TST among patients with HIV infection in Switzerland and to measure the usefulness of isoniazid preventive chemotherapy and the incidence of tuberculosis (TB) relative to TST and CD4 cell count at the time of TST. DESIGN: Prospective multicentric cohort study of HIV infected patients; comparison of patients tested and non-tested, and of those with positive and negative TST. Multivariate comparison (Cox model) of TB incidence taking into account TST results and CD4 cell count at the time of TST. RESULTS: Out of 5802 patients followed up in the Swiss HIV cohort study as of September 1994, 34.6% were tested. Native Swiss were more likely to be tested than patients from high TB prevalence countries (36% vs 21% of native Africans). Patients registered after 1990 were more often tested than before (50% vs 26%). Of 2010 tested patients, 6.8% had a > 5 mm induration. African patients were more frequently TST positive (26%) than Swiss nationals (6%). Among patients with CD4 > 500 per mm3 at the time of the test, 16% were TST positive compared to 1% of those with CD4 < 200 per mm3. Out of 25 cases of tuberculosis among tested patients, 84% (21) occurred among TST-negative patients. Tuberculosis incidence among TST-negative patients with CD4 > or = 200 per mm3 at the time of the test was 0.3 per person-year. In comparison, the adjusted relative risk of tuberculosis among TST-positive patients with CD4 > or = 200 per mm3 was 5.5 (95% confidence interval [CI]: 1.2-23.9) and 6.6 (CI: 2.3-19.0) among TST-negative patients with CD4 < 200 per mm3. CONCLUSION: Despite its usefulness, TST is often performed too late and is therefore difficult to interpret. TST should be done as early as possible. TST should be done as early as possible. Among patients with CD4 < 200 per mm3, a negative TST should not preclude the use-of preventive chemotherapy, particularly in those originally from countries with a high incidence of tuberculosis.