The histological pattern, volumetric fractions (VF) of tubular and interstitial tissue and the germinal cells, and the mean tubular diameter (MTD) were studied in 175 biopsies from 134 boys aged 3.5–16.9 years with unilateral or bilateral undescended testes. The findings were compared to those obtained in autopsy material from 67 normal boys aged 0.3–20.6 years. In both groups, the qualitative histological pattern was similar to the findings in other reports. In the normal material, VF of spermatogonia A was relatively stable whereas VF of the total spermatogenesis increased sharply after 10 years of age. In the patients, the mean values of both these variables were markedly decreased in all age groups. VF of tubular tissue and the ratio VF tubular/VF interstitial tissue were relatively stable in childhood and increased after 10 years of age both in the patients and in the normal group. In patients aged 3.5–7.9 years, the testicular volume and the volume of tubular tissue per testis were decreased. The MTD of undescended testes was decreased in patients aged 3.5–5.9 years and over 10 years of age. None of the variables investigated showed significant differences between cases with high (intraabdominal and canalicular) and low retention. The results indicated that impairment of the germinal and Sertoli cell lines were usually present in retained testes after 3.5 years of age. The normal maturational changes of tubular and interstital tissues were retarded or absent in pubertal crypt