The normal histological findings of the testicles with simultaneous azoospermia according to the former opinion renders greatly probable an obstruction of the seminal ducts. The reversed case of an ascertained obstruction, however, must not necessarily contain also normal histological structures or normal hormone tests. In the obstruction syndrome in 6 out of 23 cases we find changes in the diameter of the tubule, reductions of the spermatogenesis and changes at the interstice. Imprint-cytologically an increase of pathological spermatides at the cost of normal cell developments is shown. According to this the FSH-values are slightly but statistically significantly increased. LH, PRL and testosterone are found within the normal. For the diagnostic delimitation of the obstruction syndrome from a production azoospermia according to these findings the performance of a biopsy of the testicles with histological evaluation and the determination of the FSH-value. The relatively discrete tubular changes alone cannot be used for the explanation of the discrepancy between anatomical and functional results of the operative reconstruction of the seminal ducts.