Numerous trials were conducted during the past decades to evaluate the nutritive value of flax seed and oil for lactating dairy cows. The current meta-analysis was undertaken to assess the overall impact of different forms of flax oil, seed or fractions of seed on production performance and transfer efficiency of its constituent 18:3 n-3 from diet to milk. The database was composed of 85 research papers published between 1998 and 2015 for a total of 158 dietary treatments. Supplements were classified as (1) free oil, (2) intact untreated whole seed (WS), (3) mechanically processed WS (rolling or grinding), (4) extruded WS, (5) protected flax (oil or WS; encapsulation, xylose or formaldehyde treatments, Ca salts), and (6) flax hulls. A Wilcoxon nonparametric test was used to compare these 6 categories and significance was declared at P < 0.05. Feeding mechanically processed WS (31.7 kg/d^a) increased the yield of energy-corrected milk (ECM) as compared with intact WS (26.2 kg/d^b), extruded WS (28.1 kg/d^b), flax hulls (25.5 kg/d^b), and free oil (25.4 kg/d^b), whereas intermediate ECM was observed with protected flax (28.9 kg/d^ab). The feed efficiency (ECM/DMI) was improved by feeding mechanically processed WS (1.55 kg/kg^a) as compared with all other flax forms. Among these remaining categories, feed efficiency was greater for extruded WS (1.35 kg/kg^b) and free oil (1.35 kg/kg^b), intermediate with intact WS (1.28 kg/kg^bc) and protected flax (1.39 kg/kg^bc), and lower with flax hulls (1.24 kg/kg^c). The transfer efficiency of 18:3 n-3 from diet to milk was, from the lowest to the highest, of 1.96%^d, 3.63%^c, 4.17%^bc, 4.79%^ab, 5.80%^a, and 6.50%^abc for free oil, intact WS, extruded WS, flax hulls, mechanically processed WS, and protected flax, respectively. To conclude, mechanically processed WS was the most suitable treatment, among the 6 flax forms evaluated, to combine optimum lactation performance and protection of 18:3 n-3 to increase its transfer into milk fat.