Résumé : In humans and animal models, females express higher immune reactivity and more robust inflammatory responses. We analyzed the expression of current inflammatory markers in 149 children (74 girls and 75 boys) with three chronic inflammatory diseases: 50 with asthma, 47 with cystic fibrosis, and 52 with sickle cell anemia to evaluate the potential differences in clinical response according to sex. Data including temperature, neutrophil count (NC), and C-reactive protein were recorded for each patient at several time points according to his/her disease. In asthma, NC was higher in girls than in males (P < 0.02), as were doses of cortisone (P < 0.04) or inhaled bronchodilators (P < 0.01) received at recovery. In cystic fibrosis, NC became significantly higher in girls at age 5 years (P < 0.003), whereas episodes of infection and antibiotic administration were already significantly more frequent in girls at age 2 years (P < 0.02 and P < 0.05, respectively). In sickle cell anemia, the number of crises since diagnosis and number of acute chest syndrome episodes were significantly higher in girls (P < 0.01 and P < 0.05, respectively). Our study extends the documentation of a relationship between sex, inflammatory markers, and clinical outcome in prepubescent children, suggesting a genetic predetermination is more likely than hormonal influence.