Real time ultrasonography was used to compare the prevalence of gall stones in two groups of women aged 40-69: 632 women recruited from general practice registers and 130 vegetarians. One hundred and fifty-six (25%) of the 632 women who ate meat and 15 (12%) of the 130 vegetarian women either had gall stones visible on ultrasonography or had previously undergone cholecystectomy (p less than 0.01). The prevalence of gall stones was found to increase with age and body mass index. The 2.5 fold increase in risk of developing gall stones in non-vegetarians compared with vegetarians was reduced to 1.9 when controlling for these two potentially confounding factors, but remained significant. A family history of gall stones was reported more often by women with gall stones, but no association was found with parity or use of exogenous oestrogens. Thus the importance of age and obesity to determine the prevalence of gall stone was confirmed, and a dietary factor associated with vegetarianism may prevent this common condition.