We have observed marked variation in the in-vitro development of individual human embryos to the blastocyst stage which was poorly associated with the grade of the embryo on the day of transfer (day 2). Therefore we also graded embryos at the blastocyst stage to determine if their development potential can be better predicted at this stage. A total of 41 blastocysts were categorized into three grades depending on their morphology. Various parameters such as hatching, adherence, growth and amount of human chorionic gonadotrophin (HCG) secreted were documented. The number of nuclei in another 34 blastocysts belonging to the three grades were measured. Also morula stage embryos with large vacuoles but no morphological differentiation were included in a separate group called 'vacuolated morulae'. Overall, there was no significant difference between grade 1 and 2 blastocysts as assessed by their development to day 14, cumulative HCG secretion and nuclei counts. Grade 3 blastocysts reflected 'poor' morphology, secreted significantly lower levels of HCG and had significantly fewer nuclei than the other two grades. Vacuolated morulae did not resemble blastocysts in their morphological development, secreted no HCG and had significantly fewer nuclei than grade 1 and 2 blastocysts. This study illustrates the need for selecting blastocysts for transfer to the uterus or for cryopreservation to allow a better assessment of the success of these techniques. In addition, the data will be useful for evaluating the effects of techniques such as co-culture with feeder cells on the development of embryos to the blastocyst stage and beyond.