Objective: To evaluate whether levels of n-6 long chain polyunsaturated fatty acids (LCPs) in human breast milli. are related to the mother's atopic constitution, and whether a decreased level can be restored by gamma-linolenic acid supplementation. Design: Cross-sectional study and dietary supplementation trial. Subjects: 20 atopic mothers and 20 non-atopic mothers (controls), all lactating. Setting: General population. Interventions: The atopic mothers were randomly assigned to low (n = 10) or high (n = 10) dosage oral supplementation with oral borage oil for one week (230 or 460 mg gamma-linolenic acid (18:3n-6) per day). Main outcome measures: Essential fatty acid composition of the breast milli. total fat fraction, determined by gas liquid chromatography. Results: Arachidonic acid (20:4n-6) was lower in breast milk of atopic mothers compared with non-atopic mothers (0.39 wt% vs 0.46 wt%. difference -0.07% wt% (95% confidence limits -0.13, -0.01 wt%; P <0.05). The ratio between linoleic acid and the sum of n-6 derivatives did not differ between these groups, indicating no difference in delta-6-detsaturase (D6D) activity. Supplementation or the atopic mothers significantly increased the levels of gamma-linolenic acid and dihomo-gamma-linolenic acid in breast milk in a dose-related way, but the level of arachidonic acid was not increased. Conclusion: We found a decreased level of arachidonic acid in breast milk in atopic compared to non-atopic mothers. but no indication that the rate-limiting enzymatic step (D6D) is involved. Supplementation increased the precursor pool but did not restore the level of arachidonic acid. We conclude that atopy is related to a metabolic disturbance beyond the DOD enzymatic step. A low level of arachidonic acid in breast milk may be a risk factor for the development of atopy in the inf:ant, especially when the possible underlying metabolic disturbance of EFA metabolism is inherited by the child.