Purpose: To evaluate the surgical success after trabeculotomy for primary congenital glaucoma (PCG) in Denmark in the last 40 years. Methods: Retrospective review of medical files on children who all underwent trabeculotomy as the first surgical intervention between January 1rst, 1977 and December 31, 2016. Information on diagnosis and surgical procedures were extracted. Primary outcome was surgical success was defined as intraocular pressure (IOP) < 21 mmHg without medications one year after surgery. Secondary outcome was the number of reoperations needed. Results: Out of 118 children with PCG, 96 children (144 eyes) had an ab externo trabeculotomy as primary IOP lowering procedure opening Schlemm’s channel 4 clock hours into the anterior chamber. Complete surgical success at one year was achieved in 73% (106 of 144 eyes). On the long term, IOP could be controlled by one surgical procedure in 2/3 of children, eight eyes required >5 surgical procedures to control IOP. In 12.1% of children the second glaucoma procedure was performed >5 years after the initial trabeculotomy. There was a tendency towards greater risk of needing a second procedure in patients < 3 months of age at primary trabeculotomy (hazard ratio, HR, 2.01, 95% CI 0.96–4.22) and in boys (HR 2.02, 95% CI 0.97–4.18) and a lower risk of requiring a third surgery if the second surgery was trabeculectomy with MMC. Conclusion: Dedicated follow-up of patients with PCG is essential as the disease may continue to progress even after years of quiescence and some children need multiple surgeries to control IOP. Additional glaucoma procedures are required in 1/3 of children and boys and younger patients seem to be at greater risk.