A six week study from Southern Illinois University found that high-volume abdominal exercise training had no effect on bodyweight, body fat percentage, abdominal fat percentage, waist circumference or abdominal skin fold measurement. The test subjects increased abdominal endurance capacity substancially. There was no effort during the study to reduce abdominal fat through caloric restriction. The study showed that abdominal exercises without dieting had no effect on abdominal fat. If you want a six pack abs, you have to do a lot more than sit ups and leg raises.
(Journal Strength Conditioning Research)