Based on a well established association between testicular cancer and undescended testis and more recent publications on epidemiological links between these disorders and male infertility, we proposed the existence of a testicular dysgenesis syndrome (TDS). In most cases TDS presents with impaired spermatogenesis, only in rare cases the full range of its signs, including genital malformations and testicular cancer can be seen in one patient. In order to further corroborate our hypothesis about the presence of testicular dysgenesis in patients with testicular abnormalities, we decided to re-analyse recent testicular biopsies derived from patients with infertility, hypospadias and undescended testis. We searched for histological signs of testicular dysgenesis: microliths, Sertoli-cell-only tubules, immature seminiferous tubules with undifferentiated Sertoli cells, and tubules containing carcinoma in situ (CIS) cells. We identified 20 patients who fulfilled the histological criteria for testicular dysgenesis, 9 of whom were diagnosed with uni- or bilateral testicular germ cell neoplasia, and the remaining ones with subfertility. The presence of CIS was detected in 5 patients (3 of them with overt contralateral germ cell tumours). In all but one of the CIS cases, at least one additional sign of testicular dysgenesis was detected. Clinical records of all patients were subsequently analysed. The majority of cases had oligozoospermia or azoospermia. Their reproductive hormone profiles correlated with the results of semen sampling and testicular histology. In conclusion, our study of 20 patients with various reproductive abnormalities provided evidence that TDS is a real clinical entity. We speculate that most of these abnormalities are caused by adverse environmental effects rather than specific gene mutations.