Reports on drug disposition in man often overemphasize statistical analyses or correlation coefficients at the expense of clear statements on the biological meaning of an experiment. Moreover, the results of many such studies are presented as though they were immutable. In reality, the levels of statistical significance often depend on, and are intimately related to, numerous host factors. Results can fall anywhere within a broad spectrum of values determined by the host factors of the subjects selected for investigation. The investigator's objectives determine whether subjects are selected for uniformity or heterogeneity of these host factors. Since only a small number of subjects can be studied, it is unlikely that all critical host factors will be represented. Therefore, studies repeated elsewhere may not duplicate subject host factors with sufficient precision to allow a similar outcome. Thus, a full description of critical host factors is required for all subjects who serve in studies on drug disposition. This requirement is particularly important because many subjects suffering from diseases exhibit simultaneous changes in critical host factors, thereby modifying the dosage requirements for drug therapy. The number of host factors recognized as capable of influencing drug disposition has risen due to recent discoveries in this active field.Clinical Pharmacology and Therapeutics(1982)31, 1–7; doi:10.1038/clpt.1982