There is significant need for rapid yet reliable evaluation of promising new interventions for the treatment of patients with cancer or HIV infection. Considerable attention has been given to identifying replacement or 'surrogate' endpoints for the true clinical efficacy endpoints, in order to reduce the cost, size and duration of clinical trials. We discuss issues which affect the validity of surrogate markers. The reliability of the CD4 lymphocyte count marker is carefully considered in clinical trials of anti-retroviral agents in HIV infected individuals. The nature of surrogate markers and their reliability is discussed in cancer prevention, screening and treatment trials. Some suggested uses of marker information are also considered.

Acquired Immunodeficiency Syndrome/*DRUG THERAPY/IMMUNOLOGY/ MORTALITY Antiviral Agents/THERAPEUTIC USE Clinical Trials/*STATISTICS & NUMER DATA Clinical Trials, Phase III/STATISTICS & NUMER DATA CD4-Positive T-Lymphocytes/*DRUG EFFECTS/IMMUNOLOGY Follow-Up Studies Human Leukocyte Count/*DRUG EFFECTS Neoplasms/*DRUG THERAPY/MORTALITY Reproducibility of Results Survival Analysis Treatment Outcome Tumor Markers, Biological/ANALYSIS JOURNAL ARTICLE REVIEW REVIEW, TUTORIAL

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