OBJECTIVES: Dietary n-3 polyunsaturated fatty acids (PUFA) derived from fish may reduce the incidence of sudden cardiac death (SCD). The aim of the present study was to examine associations between n-3 PUFA and 24-h heart rate variability in patients with type 1 and type 2 diabetes mellitus (DM). DESIGN: Observational study. SETTING: The out-patient\'s diabetic clinic at Hjorring Hospital, Hjorring, Denmark. SUBJECTS: Forty-three patients with type 1 DM and 38 patients with type 2 DM. MAIN OUTCOME MEASURES: The patients fulfilled a food-questionnaire regarding fish consumption, whilst the content of n-3 PUFA in platelets was measured and 24-h heart rate variability (HRV) was obtained. RESULTS: The patients fish consumption was strongly related to their content of n-3 PUFA in platelets. Furthermore, in patients with type 1 DM a close positive association was found between the content of n-3 PUFA in platelets and 24-h HRV. This association was not significant in patients with type 2 DM. CONCLUSIONS: The positive association between n-3 PUFA in platelets and HRV may indicate a beneficial effect of n-3 PUFA on HRV in patients with type 1 DM. Further studies are warranted to clarify whether supplementation with n-3 PUFA reduce the risk of SCD amongst patients with DM.