Antisperm antibodies are formed as a result of vasal and epididymal obstruction. Fourteen males of different ages (pre-, peri- and post-pubertal) with bilateral congenital vasal agenesis and epididymal obstruction secondary to cystic fibrosis (CF), and seven men with congenital bilateral aplasia of the vas deferens (CBAVD) were evaluated with regard to both the presence and levels of serum antisperm antibodies, and the CF-genotype. While IgA and IgG were not detected among pre- and peri-pubertal CF patients, 4 out of 10 (40%) exhibited IgM binding to sperm tail-tip. Post-pubertal CF patients showed high antisperm antibody (ASA) levels in 3 of the 4 males (75%) evaluated for the three isotypes assayed. ASA were found in 5 of 7 CBAVD patients (71%); IgG (n = 3) and IgM (n = 4) were found to be the predominant isotypes bound to sperm tail-tip. CF-genotype analysis revealed two pre-pubertal patients with the DeltaF508/DeltaF508 CF-genotype and a positive ASA response, thus suggesting an earlier or more severe blockage. In addition, the two CBAVD patients found to have a ?/? CF-genotype on the initial screening did not have ASA. The altered antigenicity of sperm associated with initiation of spermatogenesis appears to modify the antisperm antibody isotypes. Further studies on a larger number of patients may allow for a better understanding of the ASA response, as well as a better understanding of a possible phenotype/genotype association between the CF-genotype and the immunologic response.