Objective: To evaluate the clinical phenotype, disease course, laboratory, and genetic features of patients with CLN2 disease over a 20 year period with a special emphasis on risk factors for diagnostic delay. Methods: Thirty patients (23 families) with CLN2 disease, evaluated between 1996 and 2019 in a tertiary referral center in Turkey, were included. Clinical features, diagnostic pathway, disease course, genetic data, electrophysiological, and neuroimaging findings were analyzed, retrospectively. The patients diagnosed between 1996 and 2009, and 2010-2019 were defined as group 1 (G1), and group 2 (G2), respectively. Patients in these two groups were also compared. Results: The median age at symptom-onset was 36 months (20 months-7 years). Most common presenting symptoms were seizures (70%), followed by language delay (43%), and psychomotor regression (27%). Median age at diagnosis was 5.2 years (1.6-11 years) with a median 27 months (1 month-7 years) of diagnostic delay. Age at diagnosis was earlier in G2 (4.6 years vs 7 years, p = 0.002), with a shorter time to diagnosis (21 months vs 39 months, p = 0.004). Median time between the onset of first symptoms and death was 8.3 years (SE 1.0). Electroencephalograms (EEG) revealed abnormal features predominantly in posterior hemispheral regions and a photoparoxysmal response to intermittent photic stimulation was detected in 53% of the patients. Cerebellar (96%)/cerebral atrophy (83%), and white matter changes (57%) were the most common radiological abnormalities. Conclusions: Most of our patients presented with late-infantile onset seizures. Despite increased availability of enzymatic and molecular testing, there is still a considerable diagnostic delay. (c) 2021 European Paediatric Neurology Society. Published by Elsevier Ltd. All rights reserved.