Patients with renal insufficiency often react abnormally to a number of drugs. Small doses that are safe under normal conditions may cause severe and even fatal side-effects. As a consequence, modification of the usual drug dosage of these drugs is required in renal insufficiency. Since the risk of retention concerns only those drugs which are mainly excreted by the kidney, it is possible to establish a mathematical relationship between glomerular filtration rate and the rate of drug elimination. These relationships serve as a basis for the determination of the proper dosage regimen for the individual patient. Such dosage adaptation for intermittent drug administration can be obtained by two methods and a series of compromises between them: (1) increase of the dosage interval without changing the dose, and (2) reduction of the dose without changing the frequency of administration.One must however, not only consider inadequate drug elimination but also a number of other factors. Some of these modify the behaviour of the drug, such as hypoalbuminaemia, which causes an increase of the unbound portion of the drug; anomalies of the volume of distribution, as found in patients with oedema; metabolic disturbances; alteration of absorption from the gastro-intestinal tract, etc. Other factors are related only indirectly to the pharmacokinetic behaviour of the drug. Frequently, there is an increased sensitivity to the undesirable side-effects of certain drugs in patients with renal insufficiency, causing the level of tolerance to be lowered compared with normal patients. Such an effect probably involves functional or morphological modifications of the drug receptors, or interaction with substances retained in renal insufficiency. Furthermore, drugs may accentuate the consequences of the nephropathy or have increased nephrotoxicity for those with diseased kidneys.It is with these important reservations that a critical analysis of the proposed methods of adapting drug dosage in renal insufficiency is presented. An appendix tabulates the effects of renal insufficiency on the behaviour of 117 drugs.Irrespective of the method used to calculate drug dosage, all patients with renal disease must be monitored closely, particularly for signs of unexpected drug toxicity.