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Haemodialysis patients often have sympathetic hyperactivity. The hypothesis of this study was that a switch from three times weekly to short daily dialysis could affect sympathetic hyperactivity.We studied 11 patients (eight men; aged 46 ± 8 years) stable on haemodialysis for at least 1 year before and 6 months after conversion from three times to six times weekly dialysis without increasing total dialysis time (short daily dialysis). Seven patients were restudied 2 months after switching back to three times weekly haemodialysis.Ultrafiltration volume per session decreased from 2.4 ± 1.0 to 1.5 ± 0.6 l (P < 0.05). The extracellular fluid volume (bromide distribution space) did not change. Mean arterial pressure (without medication) decreased from 113 ± 11 to 98 ± 9 mmHg (P < 0.05). Cardiac output (Doppler echocardiography) did not change, but peripheral vascular resistance decreased from 25.4 ± 6.4 to 21.2 ± 3.2 mmHg per min/l (P < 0.05), in conjunction with a decrease in muscle sympathetic nerve activity (MSNA) from 39 ± 19 to 28 ± 15 bursts/min (P < 0.05). Ambulant 24 h blood pressure decreased and the nocturnal blood pressure dip increased during short daily dialysis. The seven patients who were switched back to alternate day haemodialysis showed a return of the high MSNA and peripheral vascular resistance.The study shows that sympathetic hyperactivity in haemodialysis patients is reduced by increasing the frequency of treatment sessions. This is probably because of the decrease in number or magnitude of the fluid fluctuations.