Circulating interleukin-1 receptor antagonist (IL-1 Ra) levels have been shown to reflect disease activity in certain conditions in adults. We determined circulating IL-1Ra references values for healthy neonates (healthy preterms and term infants with mild disease only) on days 2 (n = 17) and 4 of life (n = 23). Mean gestational age was 35 +/- 2.6 weeks. On the 2nd day of life IL1-Ra levels were 0.78 ng/ml (0.49/2.65), on day 4 0.38 ng/ml (0.20/0.48) (median, 25th/75th percentile, P = 0.01). The values were not influenced by gender. In neonates with severe illness (septicaemia, asphyxia, neonatal respiratory distress syndrome), who received invasive intensive care, circulating IL-1Ra levels were significantly higher than in the reference group of healthy newborns. On the 2nd day of life (14.72 ng/ml (4.38/18.67) versus 0.78 ng/ml (0.49/2.65), P < 0.0001; on day 4 of life, 3.38 ng/ml (0.80/11.99) versus 0.38 ng/ml (0.20/0.48), P < 0.005 (values are median; 25th/75th percentile, Mann-Whitney U-Wilcoxon Rank Sum W Test, two-tailed P).Compared to healthy individuals beyond the neonatal period, Il-1Ra concentrations are physiologically elevated within the first days of life and decline to low levels within days. In contrast, IL-1Ra levels are strikingly elevated in sick neonates.