To examine the relationship of early human embryonic development with the leve l of reactive oxygen species (ROS) in the culture media on the first day (day 1 ROS) after insemination. A prospective study. Patients undergoing assisted repro duction in a teaching hospital. Patients undergoing conventional IVF (n = 104; 1 15 cycles) and intracytoplasmic sperm injection (ICSI) (n = 91; 96 cycles) were included. Both fertilization and early cultures were performed in human tubal fl uid with 5%serum substitute supplement. Day 1 ROS levels in the central well (s ample) and the outer well (control) of each embryo culture dish were measured af ter overnight incubation by chemiluminescence assay using luminol as the probe. Fertilization rate and embryo quality at day 3 and 5 were recorded for each cycl e. Age, parity, and demographic features were also compared. High day 1 ROS leve ls in culture media were associated with low blastocyst rate, low fertilization rate, low cleavage rate, and high embryonic fragmentation with ICSI but not with conventional IVF. High day 1 ROS levels in culture media were associated with l ower pregnancy rates in both IVF and ICSI cycles. Reactive oxygen species genera ted in culture media by day 1 may be an important biochemical marker for early e mbryonic growth. Increased embryonic fragmentation and slow cleavage rate may be partially attributed to early exposure of embryos to high ROS levels in ICSI cy cles. Differential growth of ICSI embryos incubated under identical conditions m ay be in part due to differences in ROS levels of the culture medium surrounding these embryos. To examine the relationship of early human embryonic development with the leve l of reactive oxygen species (ROS) in the culture media on the first day (day 1 ROS) after insemination. A prospective study. Patients undergoing assisted repro duction in a teaching hospital. Patients underwent conventional conventional IVF (n = 104; 1 15 cycles) and intracytoplasmic sperm injection (ICSI) (n = 91; 96 cycles) were included. Both fertilization and early cultures were performed in human tubal fl uid with 5% serum substitute supplement. Day 1 ROS levels in the central well (s ample) and the outer well (control) of each embryo culture dish were measured af ter overnight incubation by chemiluminescence assay using luminol as the probe. Fertilization rate and embryo quality at day 3 and 5 were recorded for each cycl e. Age, parity, and demographic features were also compared. High day 1 ROS leve ls in culture media were associated with low blastocyst rate, low fertilization rate, low cleavage rate, and high e mbryonic fragmentation with ICSI but not with conventional IVF. High day 1 ROS levels in culture media were associated with l ower pregnancy rates in both IVF and ICSI cycles. Reactive oxygen species genera ted in culture media by day 1 may be an important biochemical marker for Early e mbryonic growth. Increased embryonic fragmentation and slow cleavage rate may be partially attributed to early exposure of embryos to high ROS levels in ICSI cy cles. Differential growth of ICSI embryos incubated under the conditions of a be be part due to differences in ROS levels of the culture medium surrounding these embryos.