Background/aims. Ideal time needed for arteriovenous fistula (AVF) maturation is still. controversial. In this study, we aimed to investigate the natural course of AVF maturation and also investigated the factors affecting AVF maturation. Methods. We studied 31 (21M/10F, mean age 55.8 +/- 16.2) chronic renal failure patients. We evaluated the patients with color Doppler ultrasound examination before the fistula operation, at the first day, and at the first, second, third, and sixth months. Radial artery (RA) diameter, flow velocity, flow, resistance index, fistula vein diameter, flow velocity, and flow were measured. Results. Patency rates at the first post-operative day and the sixth month,here 87.1% and 67.1%, respectively. Cephalic vein flow was 451.2 +/- 248.6 mL/min at the first month and 528.6 +/- 316.5 mL/ min at the sixth month. Baseline RA diameter was lower in failing fistulas than that of patent fistulas. Failing fistulas were more common in women. Conclusion. Blood flow was enough for hemodialysis at the end of the first month. However, fistula maturation had continued until the end of the study; women and patients with low RA diameter are particularly prone to fistula failure. Therefore, especially in these patients, AVF must be created at least three or four months before the predicted hemodialysis initiation time.