BACKGROUND AND METHODS: The characteristics and frequency of clinical problems with the performance of euthanasia and physician-assisted suicide are uncertain. We analyzed data from two studies of euthanasia and physician-assisted suicide in The Netherlands (one conducted in 1990 and 1991 and the other in 1995 and 1996), with a total of 649 cases. We categorized clinical problems as technical problems, such as difficulty inserting an intravenous line; complications, such as myoclonus or vomiting; or problems with completion, such as a longer-than-expected interval between the administration of medications and death. RESULTS: In 114 cases, the physician's intention was to provide assistance with suicide, and in 535, the intention was to perform euthanasia. Problems of any type were more frequent in cases of assisted suicide than in cases of euthanasia. Complications occurred in 7 percent of cases of assisted suicide, and problems with completion (a longer-than-expected time to death, failure to induce coma, or induction of coma followed by awakening of the patient) occurred in 16 percent of the cases; complications and problems with completion occurred in 3 percent and 6 percent of cases of euthanasia, respectively. The physician decided to administer a lethal