Precocious puberty is not a typical manifestation of patients with Klinefelter syndrome (KS). However, there is an increased incidence of mediastinal germ cell tumors (M-GCT) in KS, whereas the discussion of a generally higher tumor risk in this condition is still controversial. A rare subgroup of KS patients consists of prepubertal children with precocious puberty due to human chorionic gonadotropin (hCG)-producing M-GCTs. We present clinical data on a boy with KS and sexual precocity, and summarize the published data on 12 boys with KS out of 54 cases of KS and M-GCT. Clinical report: an 8.5-year-old boy presented with signs of precocious puberty. Laboratory analyses (suppressed gonadotropins, elevated testosterone) and thoracic CT demonstrated a β-human chorionic gonadotropin (β-hCG) and α1-feto protein (α-FP) secreting mediastinal tumor. Histological analysis showed a mixed germ cell tumor comprising choriocarcinoma (CH), embryonal carcinoma (EC), mature teratoma (MT), and yolk sac tumor (YS). He was successfully treated by surgery and adjuvant chemotherapy. Epianalysis of published cases: all KS patients (n = 12), age 4–9 years, presented with precocious sexual development (PP), whereas the older ones showed thorax-associated symptoms, mainly chest pain, dyspnea, and cough. The histological distribution was also age-dependent with mixed germ cell tumors predominantly in younger patients. Thus, M-GCTs are strongly associated with precocious puberty in young boys with KS. Therefore, a karyotype analysis should be included in the clinical work-up of boys with precocious puberty and M-GCT. There is still no convincing explanation for the association of M-GCTs and KS. © 2006 Wiley-Liss, Inc.