The most common way of determining who is obese, body mass index or B.M.I., may not be the most accurate in determining the risk of cardiovascular disease.

A study in the Nov. 5 issue of Lancet, the medical journal, has found that waist-to-hip ratio is a better predictor of heart attack.

A waist-to-hip ratio (waist measurement divided by hip measurement) below 0.85 in women or 0.9 in men is average. Anything above that is a risk for heart disease.

The researchers, led by Dr. Salim Yusuf, a professor of medicine at McMaster University near Toronto, studied 12,461 people who had had a first heart attack and compared them to a matched group of 14,637 without heart disease.

A body mass index greater than 28.2 in women or 28.6 in men did indicate an increased risk of heart attack, but the relationship disappeared after adjusting for age, sex, geographic region and tobacco use.

Waist-to-hip ratio, on the other hand, showed a continuous relationship to heart attack risk even after adjusting for other risk factors. Those in the highest fifth were 2.52 times as likely to have a heart attack as those in the lowest fifth.

"I don't want to tell people to abandon B.M.I. if this will make them uncomfortable," Dr. Yusuf said, "but that is what we are doing in our studies, at least in terms of risk assessment."

Waist-to-hip ratio was a predictor of heart attack even in people regarded as very lean, those with body mass indexes under 20. Also, there was no evidence of a threshold where the risk would level off: the higher the waist-to-hip ratio, the higher the risk of a heart attack.