A questionnaire was designed to determine and compare the extent of knowledge of 40 continuous ambulatory peritoneal dialysis (CAPD) and 40 hemodialysis (HD) patients about their medicines. It was administered orally to outpatients attending a clinic by an investigator unknown to the patients. Data were collected pertaining to the following target drugs: calcium supplements, phosphate binders, vitamins B and C and folic acid. HD and CAPD groups were closely matched for age and sex. Sixty percent of CAPD and 49% of HD patients were taking four or more medications. The dose of folic acid was accurately recalled by 91% and 71% of CAPD and HD patients. Indications for calcium supplements and phosphate binders were known by only 8% and 30% of CAPD patients compared to 72% and 73% of HD patients. More HD patients knew the indications for all target drugs (p less than 0.01). More CAPD than HD patients knew the expected duration of therapy (p less than 0.01). There was a trend between decreased knowledge of whether the medication was working with increasing age of CAPD patients. Most drug information was supplied by physicians, but many CAPD patients (21%) obtained information from lay sources. When given the opportunity to ask questions about their medicines, only 56% CAPD and 46% HD patients did so. The knowledge of CAPD and HD patients studied was grossly deficient in terms of indications, effectiveness, duration and action to be taken if doses were missed.