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Prospective analysis of medical files of 20 patients diagnosed with KS between 2016 and 2022. Assessed data included analysis of causes of referral, ultrasound, and clinical characterisation with hormonal evaluation of serum FSH, LH, testosterone, inhibin B, and anti-M\u00fcllerian hormone. Non-mosaic Klinefelter syndrome (47, XXY) was diagnosed in 65% of cases (13\/20) by the geneticist (including 7 cases prenatally), in 25% (5\/20) by the endocrinologist and in 10% (2\/20) by the hematologist. Ultrasound assessment revealed bilateral testicular microlithiasis (TM) in all patients. The youngest KS patient with TM was 3\u00a0months old. TM patterns have not changed during follow-ups of up to 6\u00a0years in any of the patients. In all KS patients markedly reduced echogenicity and in pubertal KS patients, also irregular echostructure of the testes was observed. The hormonal patterns observed in the study group were typical for those already described in KS. Sertoli and Leydig cell function was intact in prepubertal patients and deteriorated after the start of puberty.<\/jats:p><jats:p><jats:italic>Conclusion<\/jats:italic>: Although the degenerative process in the testicular tissue starts very early in the testes in KS and is reflected in morphological changes seen in ultrasonography, Sertoli and Leydig cell hormonal function is normal in prepubertal KS patients.\n<jats:table-wrap><jats:table><jats:tbody>\n                    <jats:tr>\n                      <jats:td colspan=\"2\"><jats:bold>What is Known:<\/jats:bold><jats:italic>\u2022 So far, normal Leydig and Sertoli cell function was observed in infants and prepubertal KS patients.<\/jats:italic><\/jats:td>\n                    <\/jats:tr>\n                    <jats:tr>\n                      <jats:td colspan=\"2\"><jats:bold>What is New:<\/jats:bold><jats:italic>\u2022 The morphological changes in the testes in KS may already be seen in early infancy.<\/jats:italic><\/jats:td>\n                    <\/jats:tr>\n                  <\/jats:tbody><\/jats:table><\/jats:table-wrap><\/jats:p>","DOI":"10.1007\/s00431-022-04663-w","type":"journal-article","created":{"date-parts":[[2022,10,25]],"date-time":"2022-10-25T08:04:23Z","timestamp":1666685063000},"page":"225-235","update-policy":"http:\/\/dx.doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Testicular microlithiasis in paediatric patients with Klinefelter syndrome from infancy till adolescence: early start of degenerative process in the testes\u2014preliminary results"],"prefix":"10.1007","volume":"182","author":[{"given":"Dominika","family":"Janu\u015b","sequence":"first","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ma\u0142gorzata","family":"W\u00f3jcik","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jerzy B.","family":"Starzyk","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"297","published-online":{"date-parts":[[2022,10,25]]},"reference":[{"key":"4663_CR1","doi-asserted-by":"publisher","first-page":"615","DOI":"10.1210\/jcem-2-11-615","volume":"2","author":"HF Klinefelter","year":"1942","unstructured":"Klinefelter HF, Reifenstein EC, Albright F (1942) Syndrome characterized by gynaecomastia, aspermatogenesis without a Leydigism and increased excretion of follicle-stimulating hormone. 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