The Sperm Chromatin Structure Assay (SCSA) was first described in the December 5, 1980 issue of Science (Evenson et al, 1980a). The data described in that article showed a significant difference between proven fertile and subfertility or infertility in men and bulls and the susceptibility to denaturation of their sperm nuclear DNA. A subsequent study (Evenson et al, 1984) described SCSA data obtained from men with testicular cancer. The data were heterogeneous, and begged the question, what was SCSA measuring? Great effort has been spent over the past 15 years on animal model systems, mostly related to dose-response toxicology experiments and large animal fertility trials. The results from these studies showed that SCSA was highly dose-responsive to toxicants, highly repeatable, and provided meaningful biological information on sperm nuclear DNA defects. The extensive data from nonhumans (Evenson et al, 1985(Evenson et al, , 1989a(Evenson et al, ,b, 1993a(Evenson et al, ,b,c, 1994Ballachey et al, 1987Ballachey et al, , 1988Evenson and Jost, 1993; Sailer et al, 1995a,b;Evenson, 1999a), and human toxicology and fertility studies (Evenson et al, 1978(Evenson et al, , 1980a(Evenson et al, ,b, 1984(Evenson et al, , 1991Evenson and Melamed, 1983;Evenson, 1999b;Larson et al, 1999Larson et al, , 2000Larson et al, , 2001 provide This work was supported in part by grant R827019 from the US Environmental Protection Agency, grants EPS-9720642 and OSR-9452894 from the National Science Foundation, and by SCSA Diagnostics, Inc. This is South Dakota Agricultural Experiment Station publication 3273 of the journal series.Correspondence to: Donald P. Evenson, Olson Biochemistry Laboratories, ASC 136, South Dakota State University, Brookings, SD 57006 (e-mail: donald evenson@sdstate.edu).Received for publication September 20, 2001; accepted for publication September 20, 2001. compelling evidence that SCSA will be useful in clinical human semen analysis (Figure 1).After reviewing current infertility center sites on the World Wide Web and approximately a dozen lay-oriented books on infertility and assisted reproductive technology (ART), it was amazing that not a single reference contained any information about the negative influence of fragmented sperm nuclear DNA on successful pregnancy outcome. Furthermore, few physicians are aware of scientific literature on this subject. Currently, if a spermatozoon has some motility as seen under a light microscope, and reasonable morphology, then many ART clinics assume that the sperm DNA must be fine. However, a number of cases have been documented in which couples have had multiple failed attempts at intracytoplasmic sperm injection (ICSI), even using donor eggs, and we found that the fragmentation of sperm DNA was above our threshold for less than 1% probability of achieving a successful pregnancy. Spermatozoa with defective DNA can fertilize an oocyte, produce high-quality early stage embryos, and then, in relationship to the extent of DNA damage, fail in producing a successful term ...