BACKGROUND: During the last decades, there has been concern that exposure to endocrine disruptors, such as persistent organochlorine pollutants (POPs), may contribute to sex ratio changes in offspring of exposed populations. METHODS: To investigate whether exposure to 2,2'4,4'5,5'-hexachlorobiphenyl (CB-153) and dichlorodiphenyl dichloroethene (p,p'-DDE) affect Y:X chromosome proportion, semen of 149 Swedish fishermen, aged 27-67 years, was investigated. The men provided semen and blood for analysis of hormone, CB-153 and p,p'-DDE levels. The proportion of Y- and X-chromosome bearing sperm in semen samples was determined by two-colour fluorescence in situ hybridization (FISH) analysis. RESULTS: Log transformed CB-153 as well as log transformed p,p'-DDE variables were both significantly positively associated with Y chromosome fractions (P-values = 0.05 and