SummaryComplement activity was compared in 50 low birth weight infants divided into appropriate and small for gestational age groups; the influence of birth weight and gestational age on complement development was also investigated. CH50and kinetics (tH50) of both classical and alternative pathway activity of complement, C3, and Factor B levels were significantly higher in small for gestational age infants (classical pathway CH50, 630 HU/ml ± 184 SD; CP tHjo, 77 min ± 47; aternative pathway CH50, 44.8 HU/ml ±11.3; AP tH50, 56 min ± 43; C3, 73.98 mg/dl ± 12.68; and Factor B, 13.17 mg/dl ± 3.67) than in weight-matched appropriate for gestational age infants (CP CH50, 523 HU/ml ± 152; CP tH50, 105 min ± 49; AP CH5o, 38.8 HU/ml ± 13; AP tHs50, 90 min ± 53; C3, 58.14 mg/dl + 9.43; and Factor B, 9.32 mg/dl ± 1.73). Complement values were lower in low birth weight infants than in adult controls (P<0.001 in all cases). All complement parameters were mainly correlated with gestational age; CH50values of the classical and alternative pathways were also highly correlated with each other (r= 0.64; P<0.001).Low birth weight infants, especially preterm infants, have an important defect of complement activity. Complement factors increase gradually during gestation and intrauterine growth retardation does not affect complement development. Classical and alternative complement pathway activities have a similar development pattern.