Purpose: To evaluate different pre-operative variables on the success of combined trabeculotomy–trabeculectomy (CTT) surgery in patients with primary congenital glaucoma (PCG) to predict those at higher risk for surgical failure and for proper parent counseling. Patients and Methods: Sixty-three eyes of patients with PCG were treated either with CTT without augmentation, or CTT augmented with mitomycin-C (0.2 mg/mL) in both subconjunctival space and under scleral flap for 3 minutes, or with CTT augmented with a collagen implant under both the scleral flap and the conjunctiva. Cases showed surgical failure was reported and evaluated in relation to different pre-operative variables. Results: Complete success (IOP ≤21 mmHg) was achieved in 52 cases (82.5%). Cumulative success probability was calculated using Kaplan-Meier survival analysis, proving that higher pre-operative intraocular pressure (IOP) was associated with higher failure rates (28.6% for pre-operative IOP ≥30 mmHg versus 4.8% for IOP <30 mmHg), with P value = 0.007. Conclusion: CTT is an effective surgical intervention in PCG patients without sight threatening complications. Univariate survival analysis showed higher rates of surgical failure in patients with higher pre-operative IOP, while other pre-operative variables were irrelevant.