APOA5 encodes a novel apolipoprotein (apo A‐V) which appears to be a modulator of plasma triglyceride (TG). In apoA5 knock out mice plasma TG level increases almost fourfold, whereas in human APOA5 transgenic mice it decreases by 70%. Some SNPs in the APOA5 gene have been associated with variations in plasma TG in humans. In addition, hypertriglyceridaemic (HTG) patients have been identified who carried rare nonsense mutations in the APOA5 gene (Q139X and Q148X), predicted to result in apo A‐V deficiency. In this study we report a 17‐year‐old male with high TG and low high density lipoprotein cholesterol (HDL‐C), who at the age of two had been found to have severe HTG and eruptive xanthomas suggesting a chylomicronaemia syndrome. Plasma postheparin LPL activity, however, was normal and no mutations were found in LPL and APOC2 genes. The sequence of APOA5 gene revealed that the patient was homozygous for a point mutation (c.289 C>T) in exon 4, converting glutamine codon at position 97 into a termination codon (Q97X). Apo A‐V was not detected in patient’s plasma, indicating that he had complete apo A‐V deficiency. The administration of a low‐fat and low‐oligosaccharide diet, either alone or supplemented with ω‐3 fatty acids, started early in life, reduced plasma TG to a great extent but had a negligible effect on plasma HDL‐C. Loss of function mutations of APOA5 gene may be the cause of severe HTG in patients without mutations in LPL and APOC2 genes.