Strong correlations have been reported between drug concentrations at steady-state and a single drug concentration determined sometime after an initial dose for lithium, nortriptyline, imipramine, desipramine, choramphenicol and theophylline. The mathematical basis of these relationships suggests that a one point method for predicting steady-state drug concentrations and individual dosing requirements should be widely applicable to most drugs and should be valid for patients having a wide range of drug half-lives.A method is presented for evaluating the optimum time of blood sampling to determine a drug concentration in serum or plasma that best correlates with steady-state levels and for defining the range of drug half-lives beyond which the predictive approach is likely to give poor results.