Objective: To investigate whether granulocyte colony-stimulating factor (G-CSG) affects endometrial thickness, implantation rates, and clinical pregnancy rates in routine, unselected IVF cycles. Design: Registered, individually randomized, two-group, parallel double-blinded placebo-controlled clinical trial. Setting: Academically affiliated private clinical and research center. Patient(s): 141 consecutive, unselected, consenting women with no history of renal disease, sickle cell disease, or malignancy who were undergoing IVF. Intervention(s): Sealed, numbered, opaque envelopes assigned 73 patients to receive G-CSF (Filgrastim, Amgen, 300 mu g/1.0 mL) and 68 to receive placebo (saline). Main Outcome Measure(s): Endometrial thickness, clinical pregnancy, and embryo implantation rates. Result(s): The mean age for the whole study group was 39.59 +/- 5.56 years (G-CSF: 39.79 +/- 5.13 years; placebo: 39.38 +/- 6.03 years). Endometrial thickness statistically significantly increased over the 5-day observation period for the whole group by approximately 1.36 mm. The increase in the G-CSF group was not statistically significantly different from the control group. Statistical models looking at treatment effects on clinical pregnancy and implantation rates demonstrated no effect of G-CSF treatment. There were no adverse events for either treatment group. Conclusion(s): In normal IVF patients, G-CSF does not affect endometrial thickness, implantation rates, or clinical pregnancy rates. Because these results were obtained in an older patient population, they may not necessarily apply to younger women.