The general population is exposed to phthalates through consumer products, diet, and medical devices. The present study explored whether phthalates, reproductive toxins in laboratory animals, were associated with altered sperm movement characteristics in men. Two-hundred twenty subjects provided a semen sample for computer-aided sperm analysis (CASA) and a urine sample for measurement of phthalate monoesters, monoethyl (MEP), monobenzyl (MBzP), mono-n-butyl (MBP), mono-2-ethylhexyl (MEHP), and monomethyl (MMP). Three CASA parameters, straight-line velocity (VSL), curvilinear velocity (VCL), and linearity (LIN), were used as measures of sperm progression, sperm vigor, and swimming pattern, respectively. There were suggestive dose-response relationships (shown as the predicted change in mean sperm motion parameter for the second and third tertiles compared with the first tertile; P value for trend) for MBzP with VSL (-2.36 microm/s, -2.81 microm/s; P =.09) and VCL (-1.67 microm/s, -2.45 microm/s; P =.4). There were suggestive negative associations between MBP and VSL (-3.07 microm/s, -2.87 microm/s; P =.08) and VCL (-3.25 microm/s, -3.46 microm/s; P =.2), and between MEHP with VSL (-1.09 microm/s, -2.73 microm/s; P =.1) and VCL (-0.29 microm/s, -2.93 microm/s; P =.3). In contrast to the other phthalates, MEP was positively associated with VSL and VCL but negatively associated with LIN. No consistent relationship was found for MMP and any sperm motion parameter. Although we did not find statistically significant associations, trends between CASA parameters, sperm velocity, and forward progression, and increased urinary levels of MBP, MBzP, and MEHP warrant further follow-up.