Changes in microcirculation have been recognized as central to many disease processes. The aim of this study was to evaluate factors, which influence the microcirculation of the skin during the first month of life in premature infants. Red blood cell (RBC) velocity, vessel diameter, and functional small vessel density (FSVD) were measured daily for the first 30 d on the upper arm in preterm infants with gestational age Ͻ30 wk. Orthogonal polarization spectral (OPS) images were analyzed off-line with the CapiScope-Image program. In 25 infants, FSVD decreased significantly from week 1 (mean Ϯ SD 236 Ϯ 33 cm/cm 2 ) to week 4 (207 Ϯ 30 cm/cm 2 ) and correlated directly with Hb levels and incubator temperature. Vessel diameters and RBC velocity did not change significantly, nor did clinical parameters such as blood pressure, heart rate or body temperature. Microvascular parameters were not dependent on gestational or postnatal age. The microcirculation of the skin might be an easily accessible window to obtain better understanding of circulatory changes in the postnatal period. Our data are essential as basis for further studies in this field. Hb levels and possible incubator temperatures have a substantial influence on functional small vessel density and therefore need to be taken in account. D isturbances of the microcirculation play a key role in many disease states (1-4). The recent development of new technologies has helped to investigate these changes in adult patients with sepsis (5-7). Previous studies have shown that parameters of microcirculation such as microvessel diameter, red blood cell velocity, and functional small vessel density (FSVD) can be measured in the skin of term and preterm infants by Orthogonal Polarization Spectral (OPS) imaging in the first week of life (8) and that FSVD increases after elective blood transfusion in anemic neonates (9). The microcirculation of the skin plays an important role in maintaining a constant body temperature and in regulating the fluid balance (10,11). Adequate function of the microcirculation is a prerequisite for tissue nutrition and oxygen supply (12). The microcirculation of the skin in neonates differs in several aspects from that of an adult. The regular architecture has been found to be poorly developed in the newborn (13). At birth, the skin shows a disorderly capillary network and no papillary loops in almost all areas, except the palms, soles, and nail folds. The skin is richly supplied by a dense subepidermal plexus demonstrating relatively little regional variation (14).Functional capillary density (FCD) is one of the parameters that delineate the microcirculation. FCD is defined as the length of red cell-perfused capillaries per observation area and is given as cm/cm 2 . FCD has been used as an indicator of the quality of tissue perfusion (15). In neonates capillary vessels, arterioles, and venules cannot clearly be differentiated in the OPS images, so that the expression FSVD is used.The aim of the study was to determine whether tissue pe...