Several studies have aimed to unravel the contribution of different types of dietary fatty acids to human health and disease. Investigations have consistently shown that high consumption of industrially produced trans-fatty acids from partially hydrogenated vegetable oils is harmful to human health, in particular cardiovascular health. Therefore, the U.S. Food and Drug Administration announced that partially hydrogenated oils are no longer 'generally recognized as safe', and trans-fatty acids are not permitted in the U.S. food supply. On the other hand, recent studies analyzing the association between circulating trans-fatty acids and disease have revealed that some ruminant-specific trans-fatty acids are associated with a reduction in incidence of disease. In this special report, we highlight recent findings and point out perspectives for future studies on this topic.