The human milk composition may be influenced by several factors, such as gestational age or genetic characteristics and dietary habits of different populations. To analyze the total lipid and fatty acid contents of human milk, we have conducted two studies, one on mothers who had delivered preterm and term newborns and another on mothers from two different sociocultural backgrounds (Spain and Panama), The total lipid content (g/100 g wet weight) was significantly higher in term (2.76 +/- 0.66; mean +/- SD) than in preterm mature milk (1.06 +/- 0.4). The relative amount of 18:1 n-9 was significantly higher in preterm than in term milk for transitional and mature milk, whereas that for the colostrum followed the opposite trend, Concerning the comparison between milk from mothers born in different countries, the relative contents of each of the fatty acids 16:0, 16:1 n-7, 18:2 n-6, 18:3 n-3, and 22:5 n-3 were higher in Panamanian than in Spanish milk, whereas the mean percentages of saturated fatty acids < 14:0, of 16:1 n-9, and of 18:1 n-9 were higher in Spanish than in Panamanian milk. Statistically significant differences were found during the three periods of lactation considered for almost all the fatty acids mentioned above, especially for 18:1 n-9 and 18:3 n-3. Although the potential biological significance of the changes in oleic acid content between preterm and term milk remains unclear, differences in fatty acid content between Spanish and Panamanian milk reflect the different composition of the diet among women from these countries.