A multicentre, double blind, randomised between-group study comparing the triglyceride lowering effect of MaxEPA, a natural marine oil, and a placebo control is described. Eighty-six patients with hypertriglyceridaemia (fasting serum triglyceride greater than or equal to 2 mmol/l) were studied for three months. There were no significant differences between the groups (48 active, 38 control) in respect of age, sex, height or weight, smoking habits or alcohol consumption. After one month triglyceride levels were reduced significantly from baseline in the treatment group and there was also a highly significant difference between the groups in favour of the marine oil. There was no significant change in serum total cholesterol in either group but there were fluctuations in high density lipoprotein (HDL) in both groups. Minor gastrointestinal side effects were reported by patients in both groups. Standard haematological and biochemical tests were done and there were no significant changes from baseline.