The study was designed to determine whether hyperviscosity of the semen sample is related to dysfunction of the male accessory glands. It was carried out on men who consecutively attended an infertility clinic between June 1989 and June 1991, and the men were grouped according to viscosity of semen samples (normal viscosity or higher viscosity). Semen samples from 229 infertility patients were studied. From these, 155 had normal viscosity and 74 showed hyperviscosity. The effect of hyperviscosity of semen samples on seminal quality and the function of the prostate was evaluated by acid phosphatase measurement, and the seminal vesicles by measurement of corrected fructose. Sperm motility (grades II-III), sperm vitality, and corrected fructose were significantly reduced in samples with high viscosity (p < 05). A high prevalence of hyperviscosity in semen samples was associated with only hypofunc-tion of the seminal vesicles. In fact, 36.5% of subjects with hyperviscosity showed reduced levels of corrected fructose. The same association with hyperviscosity was not observed when only hypofunction of the prostate was present, or when hypofunction of both prostate and seminal vesicles was present (P.NS). Further analysis showed that high viscosity is observed mainly when corrected seminal fructose levels were below 1.5 mg/mL × 10 spz/mL. It would appear that hyperviscosity affects sperm motility and is associated with hypofunction of the seminal vesicles. © 1993 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.