Klinefelter syndrome in childhood: language delay as an early warning sign for diagnosis.

Klinefelter syndrome (KS), the most common sex chromosome aneuploidy in males, is often underdiagnosed until adolescence, delaying early intervention. We describe 11 pediatric patients with KS who were followed between 2005 and 2025 to identify early markers. Three were diagnosed prenatally; the remaining eight were diagnosed at a median age of 6.1 years, mainly due to neurodevelopmental problems. Of the total of 11 patients, 9 had delayed language acquisition, followed by 8 with psychomotor delay, 5 with behavioral disorders, 3 with sleep disorders, and 2 with epilepsy. Endocrinological comorbidities were less frequent in childhood. Delayed language development emerges as a crucial early indicator. Active detection, along with other neurodevelopmental comorbidities, is essential to address underdiagnosis and enable early, multidisciplinary intervention, thereby significantly improving patients' developmental outcomes and quality of life in KS.