The issues of sexual function and fertility are becoming relevant in patients with posterior urethral valves (PUV), as more of them reach adulthood. To evaluate the semen of post-pubertal patients with PUV as a determinant of future fertility, all such patients (age >16 years) attending the follow-up urology clinic of our department from 1985 to December 1999 were contacted. Of the nine patients contacted, eight agreed to form the study group. All eight patients were asked to provide a post-masturbation semen sample and urine. Semen was analysed for pH, viscosity, liquefaction time, sperm morphology, sperm count, motility, and agglutination. The patients ages ranged from 16 to 21 years (mean 17.5 years). One patient with chronic renal failure awaiting a renal transplant refused to give a semen sample; two tried but failed to ejaculate on three consecutive visits. Their urine was negative for sperm. Of the five patients who gave semen, the liquefaction time was high in two. pH ranged from 7.2 to 8, sperm counts were 24-80 million. None of the patients had oligospermia. Abnormal sperm agglutination was present in four cases; a higher percentage of immotile sperm was also present in four. Semen abnormalities in the form of increased liquefaction time, abnormal sperm agglutination, and a high percentage of immotile sperm were thus seen in the present study. The bearing of these findings on subsequent sexual function and fertility remains a matter of speculation.