The total lipid content and fatty acid composition of preterm human colostrum and milk were analyzed from aliquots of 24-h collections of colostrum, transitional milk, and mature milk obtained from 21 women who had delivered premature infants with a mean birth weight of 1,049 +/- 38 g (mean +/- SEM) and a mean gestational age of 29 +/- 0.4 weeks. The total lipid content increased significantly with time of lactation (p less than 0.001), from 1.99 +/- 0.25 g/dl in colostrum to 3.89 +/- 0.28 g/dl in mature milk. With respect to the fatty acid pattern, the percentages of 12:0 and 14:0 increased significantly (p less than 0.001), while those of 16:0 (p less than 0.05) and 18:1 (p less than 0.001) decreased significantly with time of lactation. The percentages of 10:0, 16:1, 18:0, 18:2, and 18:3 did not vary significantly with the duration of lactation. Similar compositional changes have been described in term human colostrum and milk as the duration of lactation progresses.