Factors relating to acute blockage of modified Blalock-Taussig shunts (MBTS) have not been well described in the literature. Our aim was to determine the outcomes in patients early after undergoing MBTS operations and to identify potential risk factors for acute shunt blockage in the early postoperative period. A retrospective study was performed in a tertiary referral congenital cardiac unit. All children who underwent first shunt insertion for cyanotic congenital heart disease during the study period from 2002 to 2006 were included. Seventy-six children underwent first shunt insertion with a median age of 37 days (range 2 days-8 years) and median weight of 3.75 kg (range 2.1-17.2 kg). The shunt sizes varied from 3 to 6 mm. The rate of acute shunt blockage was 11.8% (9/76), all within the first 24 h. There were 3 early deaths (3.9%), none of which were associated with shunt blockage. The main risk factors for blockage were preoperative high hemoglobin, weight less than 3 kg, and duct patency on echocardiogram after surgery. This study identifies that high preoperative hemoglobin, a weight less than 3 kg at operation, and the presence of a patent duct are statistically significant risk factors for shunt blockage in the acute postoperative period. Further work is needed to determine if reduction in preoperative hemoglobin concentration and attempts to reduce postoperative ductal patency may alter the outcome.