In a previous experiment, the proportion of female lambs was higher when Merino ewes were offered a diet high in omega-6 (n-6) compared with low in n-6 around joining. The effect of these diets on the sex ratio of lambs born to the offspring of these ewes has not previously been investigated. The aim of the current study was to determine whether the proportion of female lambs when Border Leicester × Merino ewes were fed a diet high compared with low in n-6 around mating was affected by diet previously fed to their dams at conception. Ewes were stratified to treatment group based on the diet previously fed to their dams around mating and fed a Low n-6 (lucerne hay) or High n-6 (oats and cottonseed meal) diet for ∼40 days pre-mating and 17 days post-mating. Liveweight, fat score, plasma fatty acid status and lamb sex was determined. The ratio of n-6:n-3 fatty acids in plasma was higher (P < 0.001) and the time to oestrus shorter (P < 0.001) when ewes received the High n-6 compared with the Low n-6 diet. The proportion of female lambs was not significantly (P ≤ 0.229) higher when ewes were fed the High n-6 compared with the Low n-6 diet (59.8 vs 42.7%), but was significantly (r2 ≤ 0.78, P ≤ 0.008) positively correlated with the ratio of arachidonic acid to eicosapentaenoic acid in ewe plasma. The effect of the High n-6 diet on the sex ratio of lambs born to Border Leicester × Merino ewes was not affected by diet previously fed to dams around conception; however, results need to be confirmed with larger numbers of ewes. The effect of dam nutrition on the sex ratio of lambs and overall productivity should be monitored over several years.