Background To determine the unique clinical characteristics and treatment outcomes of diffuse infiltrating retinoblastoma (DIR). Methods This international, multicentre, registry-based retrospective case series analysed pooled data from January 2001 to December 2013, including 132 eyes from 132 patients with DIR. Results Among 2854 eyes with retinoblastoma, 132 (4.6%; 95% CI, 3.9 to 5.5) had DIR. The median age at diagnosis for DIR patients was 24 months (IQR, 15-33), with no bilateral cases of DIR. The American Joint Committee on Cancer staging showed 4.5% cT2 and 95.5% cT3 categories, with no cT1 or cT4 cases. Clinical features associated with DIR included secondary glaucoma (67%), retinal detachment (38%), diffuse vitreous seeds (37%), anterior segment involvement (24%), vitreous haemorrhage (50%) and hyphema (6%). Primary enucleation was the predominant treatment (81%), while 19% initially received systemic chemotherapy, with 6% requiring subsequent enucleation. The 5-year Kaplan-Meier survival rate for cT3 DIR was 82% (95% CI, 78 to 86), significantly lower than the 94% (95% CI, 93 to 95) for cT3 non-DIR cases (p<0.001). Cox proportional hazards regression returned a higher risk of metastatic death for DIR cT3 compared with non-DIR cT3 (HR, 3.3; 95% CI, 1.8 to 5.9; p<0.001). High-risk pathological features were more frequent in DIR (41% vs 28%, p=0.004). There was no association between DIR and local treatment failure. Conclusions Approximately 1 in 20 patients with retinoblastoma had DIR, which often presented with glaucoma, anterior segment involvement or intraocular bleeding. DIR was more lethal than non-DIR cT3, with enucleation revealing high-risk pathology.