Use of a cryostage has enabled direct observation of human spermatozoa as they are cryopreserved and thawed. Crystallization and recrystallization events are readily observed. In combination with computer-aided semen analysis (CASA) equipment it was possible to determine the consequence of altering the cooling, freezing and thawing rates of a temperature-rate profile on sperm motility. Increasing the cooling rate to 50 degrees C/min resulted in significantly lower pre-freeze to post-thaw ratios for average path velocity (VAP, 13%), mean straight line velocity (VSL, 35%), mean linearity (LIN, 28%) and straightness (STR, 24%), while the ratio of the number of cells crossing the field of view (NCF) significantly increased (30%) compared to a standard freeze-thaw temperature rate profile. The NCF pre-freeze to post-thaw ratio was associated with the percentage of cell recovery after cryopreservation. Faster thaw rates resulted in better survival of the cells, perhaps due to the shorter time during which recrystallization occurred. The NCF ratios were significantly higher (33 and 30% for thaw rates of 50 and 100 degrees C/min respectively) than for the standard profile samples. Previous studies on cell survival have shown a link between the cooling and thaw rates. The cryostage should prove invaluable in future studies to identify the causes of cryodamage to spermatozoa. When used in combination with CASA, changes to sperm function during cryopreservation can be accurately measured.