Klinefelter syndrome (KS) with the classic 47,XXY karyotype is the most frequent chromosomal aneuploidy, with a prevalence of 1 in 700 men; although the classic clinical picture is well-known and easily recognizable, most patients remain undiagnosed. The rate of diagnosis during childhood is extremely low, and only 10% of cases are identified before puberty, with a subsequent rate of ascertainment during lifetime of 25%. The low rate of timely diagnosis is because most of the classical signs and symptoms of androgen deficiency appear in mid-to late adolescence but it is important to recognize that adult men with KS may show a great variability in clinical and physical features. A common, often underappreciated, element in young boys and children with KS is the characteristic cognitive and behavioral pattern. We describe 2 patients who were diagnosed at 7.1 and 10 years through a characteristic neurocognitive profile. Both of them showed low-normal scores when evaluated by tests of general intelligence and a behavioral profile characterized by immaturity, low self-esteem, and learning disabilities. Clinical examination showed tall stature and progressive growth acceleration between 5 and 7 years, and one of them had hypoplastic scrotum with monolateral cryptorchidism. To achieve the goal of an early diagnosis of KS, it is necessary to increase medical awareness of the disease and, in particular, to augment pediatricians' knowledge that during prepubertal age pathognomonic clinical features of KS are often lacking but a characteristic cognitive and behavioral pattern is commonly present. (J Am Board Fam Med 2012;25:745-749.)