The purpose of the current study was to determine whether there were differences in the fatty acid composition of milk from vegetarian mothers compared to nonvegetarian mothers and whether fatty acid composition was related to length of time on a vegetarian diet. Median time on a vegetarian diet was 81 months (range 36-132 months). Milk fat and fatty acids produced de novo in the mammary gland did not differ between diet groups. Milk from vegetarian women (n = 12) contained higher percentages of the precursors of arachidonic acid compared to nonvegetarian women (n = 7). Although there was no difference between diet group in the amount of arachidonic acid present (percent of total), a lower percentage was observed in women the longer they were on a vegetarian diet (r = -0.724, p = 0.008), with a corresponding higher percentage of the sum of precursors of arachidonic acid (r = 0.563, p = 0.056). The percentage of docosahexenoic acid was similar in the milk of vegetarian women compared to nonvegetarian women. The length of time on a vegetarian diet had no effect on these two fatty acids. In summary, differences in fatty acid content were apparent between the two dietary groups and changes with time on a vegetarian diet suggest that not only does current maternal diet affect fatty acid milk composition, but long-term dietary habits may as well.