Background A suboptimal neurocognitive outcome has been identified in adults with early-treated PKU(ET-PKU). Episodic memory (EM) and speed of processing has been shown to be abnormal but most published studies have included small numbers of heterogenous patients. Moreover, the influence of plasma phenylalanine (Phe) on such neurocognitive functions remains uncertain. The aim of the present study was to evaluate EM and Processing Speed Index (PSI) in a large group of adult patients with ET-PKU. Methods Patients >18y, with a diagnosis of PKU from the French newborn screening program, were included from 17 centers in a national 5-y prospective cohort study (ECOPHEN). EM was assessed using the California Verbal Learning Test – 2nd Edition (CVLT-II). The speed of information processing was measured using PSI from the Wechsler Adult Intelligence Scale-Fourth Edition (WAIS-IV) and the Trail Making Test A (TMT-A). Fasting plasma Phe was concurrently measured. Data from baseline visit are presented as medians [interquartiles]. Correlations and comparisons used Pearson and t-test respectively. Results 187 adult patients with PKU were included in the study. Within the participants, 152 were classified with classic PKU (cPKU), 26 with mild PKU (mPKU) and 9 with mild hyperphenylalaninemia (mHPA). Median age was 27.2y [22.3; 32.3]. Regarding EM, cPKU patients have significantly reduced performance for immediate recall indices as compared to mPKU. No difference was observed for delayed recall indices and memory recall accuracy indices. Some significant negative correlations were identified between plasma Phe and some EM indices. After the categorical indication, which facilitates recall, the significance dissipated. After a delay, no significant correlation between long term recall performance and plasma Phe was observed. For the whole cohort, median PSI was 89 [78; 100]. A significant negative correlation between plasma Phe and PSI was identified at −0,34 [−0,46; −0,20]. PSI in patients with plasma Phe > 600 μmol was lower as compared to patients with plasma Phe < 600 μmol/L (88 ± 16 vs 97 ± 17, p = 0,006). Conclusions The present study suggests, from a large cohort, that EM is slightly influenced by plasma Phe levels in adults with ET-PKU. Our data indicates a lower immediate recall performance in patients with cPKU and significant negative correlations with plasma Phe. This result suggests a difficulty in learning information or a fragility of information encoding. When the patients are re-questioned on the learning carried out after a delay, there is no longer any significant difference according to the PKU groups and no correlations with plasma Phe. Thus, the phenomenon of memory consolidation which takes place in the interval between the encoding of information in episodic memory and its delayed retrieval appears efficient in ET-PKU patients. Regarding speed of processing, our study confirms that PSI is decreased in ET-PKU patients. Moreover, our data suggest that concurrent Phe has a negative influence on PSI. Considering the upper level of the European guidelines (600 umol/L), patients with plasma Phe > 600 μmol/L have a significant abnormality in speed of processing. This results, is in favor of an optimization of metabolic control in adults with ET-PKU.< Réduire