A number of regulatory agencies in western Europe, Japan and the United States now include guidelines for evaluating the potential immunotoxicity of chemicals, including drugs, as part of routine toxicity testing. Most testing guidelines recommend observational or functional assays that, based on studies in laboratory animals, are able to detect changes in immune function that are associated with increased susceptibility to infectious or neoplastic cell challenge. To appreciate how well observational and functional endpoints are likely to predict an increased risk of infection in humans, it is important to establish correlations between alterations in human immune function and an increased risk of disease. This review will address the clinical evidence for increased risk of disease in humans with mild to moderate levels of immunosuppression using examples from the literature, discuss specific immune system defects associated with increased rates of infection, and examine factors that impact the interpretation of clinical data. The most comprehensive data bases that address these relationships, those derived from patients with primary immunodeficiency and AIDS, are not discussed in this review. These are extreme examples of immunodeficiency and neither the specific clinical diseases that result, nor eventual outcomes, have much in common with that which occurs in individual with chronic mild-to-moderate immunosuppression.