A double-blind clinical trial to examine the effects of oral and long-acting injectable contraceptive steroids on milk lipid and its fatty acid content has been done in Szeged, Hungary, and Khon-Kaen, Thailand. In Szeged, a combined and a progestin-only pill did not significantly alter total milk lipid. In Khon-Kaen, treatment with the combined pill was followed by a significant increase in the proportion of milk lipid. In the group treated with the long-acting injectable contraceptive, depot-medroxyprogesterone acetate (DMPA), the milk lipid decreased significantly in the first six weeks in comparison with the control group and the individual pretreatment values. A similar but weaker effect was noticed with the progestin-only pill in Khon-Kaen.Few consistently significant differences were found in the shorter chain fatty acids (myristic, lauric and palmitic acids) during treatment. The percentage proportions were increased during the combined pill treatment in Szeged and Khon-Kaen, and reduced in DMPA and progestin-only treatment in Khon-Kaen milks. The possibility that these reflected a response to a milk volume-decrease in the combined pill treatment and a reduction in milk lipid synthesis in association with DMPA and the progestin-only pill is discussed. In Khon-Kaen, linoleic acid (which is not synthesized in the body) was increased by comparison with the controls, as a percentage proportion of the fatty acids in the progestin-only and DMPA groups. Calculation of the amounts of linoleic acid per litre of milk revealed that there had been a significant decrease of linoleic, eicosadienoic, dihommo-gamma-linolenic, arachidonic, docosatetraenoic acids in the w6 family and alpha-linolenic acid in the w3 family in the first two post-treatment visits (3 and 4). This reduction in essential fatty acid output follows the reduction in milk volume.In Khon-Kaen, the combined pill group showed a significant decrease compared with the controls, in the proportions of dihommo-gamma-linolenic acid. The difference in response of the mothers in Khon-Kaen and Szeged is discussed in relation to their different nutritional backgrounds.