Purpose The aim was to report cumulative incidence and time of onset of postoperative glaucoma in a paediatric early cataract surgery cohort. Methods Data were retrieved from the Pediatric Cataract Register (PECARE), a prospective register of Swedish cataract operations before 8 years of age. All eyes with surgery between January 2007 and December 2014 and a registered follow‐up were included. Cataracts caused by uveitis, trauma or coexisting congenital glaucoma were excluded. Glaucoma was defined as early onset if diagnosed within a year after surgery and late onset if diagnosed later. Results The study included 288 eyes in 207 children (106 girls), 81 with bilateral and 126 with unilateral cataracts, with a mean follow‐up of 3.31 ± 1.77 years. Of the 288, 168 (58.3%) had surgery before 3 months of age; most of these 92.3% (155/168) were defined as dense, 208 (72.2%) were below 1 year of age. Cumulative incidence of surgically treated glaucoma among individuals was 23.7% (49/207). Median time to glaucoma onset was 0.91 years (range: 0.05–4.97 years) for eyes. Early‐onset glaucoma was found in 98 % (63/64), and late onset in 2% (1/64). Conclusion In this paediatric cataract cohort, a majority of eyes had surgery before 3 months of age (58.3%). Secondary glaucoma‐onset peaked within the first postoperative year, with a cumulative incidence of 23.7%. Surgery performed after the first month of life, resulted in a lower glaucoma rate. Long‐term follow‐up will reveal whether the low rate of late‐onset glaucoma with early surgery will last, and if so, the consequences.