<p><b>OBJECTIVE</b>Leptin (LEP) is mainly produced by white adipose tissue and participates in the energy metabolism and regulation of growth. Cooperating with the other metabolic hormones, it plays an important role in the developments of fetus and neonates. This study was designed to test the serum levels of LEP, neuropeptide Y (NPY), insulin (INS) and insulin-like growth factor-1 (IGF-1) and measure the body mass index (BMI) and head circumference (HC) at different days of life of premature infants with or without serious diseases and to find the changes of serum levels of LEP as well as NPY, INS and IGF-1, the relationship between those hormones and the changes of body weight and the influences of diseases on the levels of those hormones in premature infants.</p><p><b>METHOD</b>The clinical data as well as weights, lengths, HC of 40 sick premature infants (sick group) and 30 premature infants without any diseases (control group) were collected and the serum levels of LEP, NPY, INS and IGF-1 were determined by using radioimmunoassay (RIA) at d 1, d 7 and d 12 of life. BMI was calculated by weight (kg)/length (m)(2). SPSS13.0 was used to analyze the data</p><p><b>RESULT</b>(1) In sick group the serum LEP levels were 0.74 +/- 0.21, 0.60 +/- 0.18, 0.82 +/- 0.12 (mg/L) (P < 0.01), the BMI were 9.81 +/- 1.24, 8.36 +/- 0.87, 9.08 +/- 1.12 (kg/m(2)) (P < 0.01) on d 1, d 7 and d 12, respectively. In control group serum LEP levels were 0.78 +/- 0.17, 0.71 +/- 0.17, 0.88 +/- 0.58 (mg/L) (P < 0.01), the BMI were 10.03 +/- 1.04, 9.35 +/- 0.80, 11.06 +/- 0.82 (kg/m(2)), on d 1, d 7 and d 12, respectively (P < 0.01). In both groups, serum LEP levels as well as the BMI decreased on d 7 and reincreased on d 12. The differences of serum LEP levels and BMI between sick group and control group at d1 were not significant (P > 0.05); compared with control group, the serum LEP levels and BMI on d 7 and d 12 in sick group were lower and the differences were significant. (2) There were positive correlations between serum LEP levels and BMI in sick group as well as in control group. (3) In sick group, the serum NPY levels at d 1, d 7, d 12 were 55.33 +/- 9.38, 46.64 +/- 6.17, 75.13 +/- 9.12 (ng/L) (P < 0.01), INS were 10.07 +/- 2.63, 7.71 +/- 2.77, 10.37 +/- 2.29 (mU/L) (P < 0.01), IGF-1 were 38.66 +/- 11.42, 31.98 +/- 7.34, 41.84 +/- 8.05 (mg/L) (P < 0.01), respectively. In control group, the serum NPY levels at d1, d 7 and d 12 were 57.77 +/- 7.15, 48.49 +/- 8.81, 81.36 +/- 8.51 (ng/L) (P < 0.01), INS were 11.55 +/- 1.99, 8.28 +/- 2.87, 15.42 +/- 3.80 (mU/L) (P < 0.01), IGF-1 were 37.76 +/- 7.07, 34.33 +/- 8.97, 50.19 +/- 8.38 (mg/L) (P < 0.01), respectively. In both groups, serum levels of NPY, INS and IGF-1 had positive correlations with serum LEP levels as well as BMI on the corresponding days and decreased on d 7 and reincreased on d 12.</p><p><b>CONCLUSION</b>(1) The serum LEP levels decreased on 7 d of life and reincreased on 12 d of life, which corresponded to the changes of the physical development of premature infants. (2) The serum LEP levels in sick premature infants decreased definitely as compared with control group, which suggested that diseases had negative influences on the LEP levels and the physical developments were slowed down in sick premature infants. (3) The serum levels of NPY, INS and IGF-1 had positive correlations with LEP levels as well as BMI at the early period of life, which suggested that NPY, INS and IGF-1, cooperating with LEP, might take part in the regulation of development of premature infants.</p>