Plasma fibronectin is an attachment protein important for maintaining capillary integrity and host defense mechanisms. Depletion of plasma fibronectin has been shown to occur in adults after septic shock, major trauma, and burns. Limited laboratory and clinical studies suggest a correlation between decreased plasma fibronectin levels and increased pulmonary capillary permeability and tissue perfusion. Mild and transient plasma fibronectin depletion has been observed in adults after cardiovascular operations. We measured plasma fibronectin by immunoturbidometric assay in 20 children (age 6 months to 12 years) undergoing repair of congenital heart defects. Plasma fibronectin levels immediately after operations and daily thereafter were compared with the preoperative values. Plasma fibronectin declined on postoperative days 1, 2, 3, 4, and 5 (p < 0.05). A nadir was reached on day 3 with a tendency toward recovery thereafter. Patients with a therapeutic intervention score of more than 35 had greater magnitude of plasma fibronectin decline than those with a score of less than 35 at 24 hours after the operation (p < 0.005). We conclude that (1) significant and prolonged plasma fibronectin depletion occurs after cardiovascular operations in children; and (2) postoperative plasma fibronectin depletion is associated with increasingly complex surgical intervention. Reduced plasma fibronectin synthesis and more extensive operations for congenital heart defects are likely reasons for children being more susceptible than adults to plasma fibronectin depletion after cardiovascular operations.