Between May 29 and September 13,1991,4 patients developed acute intravascular hemolysis during hemodialysis with Monitral-S delivery systems and Hospal BSM A77 blood lines. All had malaise, nausea and headache; 3 had severe abdominal pain and 2 became very ill. Plasma hemoglobins were 3-21 g/l and LDH 542-3,300 IU in the 4 patients. Hepatoglobin became unmeasurable in 3 and was 0.09 g/l in the 4th patient. Soon afterwards, we found the arterial blood line tightly kinked at the dialyzer inlet port in the 4th patient and released it; he developed abdominal pain, hemolysis was present. We then found these lines had an extra long pump segment, and the rest was short and fitted poorly. When put in the first tubing organizer, severe kinking could occur just after the pump segment, causing back pressure but no alarm. We produced early visible hemolysis in a 1-liter circulating closed loop blood system with the blood line kinked either at the dialyzer inlet or just below the first arterial line tubing organizer with 40 g/l free plasma hemoglobin by 30 min. We excluded reported causes of intravascular hemolysis during hemodialysis. No hemolysis occurred before or during the 9 months after we discarded BSM A77 lines. The evidence indicates that kinked blood lines caused the hemolysis.