This study was designed to determine the effects of high semen viscosity on sperm chromatin stability. Semen samples obtained from men with normal and high viscosity were studied. Sperm chromatin stability was tested by exposure to sodium dodecyl sulfate (SDS) only and SDS together with a zinc-chelating agent, disodium ethylene diamine tetraacetate (SDS+EDTA). After SDS incubation, stable sperm was 61.36 ± 3.0 and 54.71 ± 3.42% for normal and high semen viscosity, respectively (P:NS), and after SDS+EDTA, it was further reduced to 12.48 ± 0.99% in semen samples with normal consistency and in a less magnitude in semen samples with high viscosity (25.6 ± 5.2). Comparing values obtained in SDS+EDTA, a high sperm stability was observed in samples with hyperviscosity (p < 02). In samples with normal viscosity the percentage of grossly swollen sperm increased 5.40 times from the values obtained in sperm incubated with SDS to the values obtained with SDS+EDTA, whereas in samples with high viscosity the percentage increased only 2.2 times. It is concluded that hyperviscosity is associated with a high sperm chromatin stability in situations when a zinc-chelating agent is present. © 1994 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.