It has been suggested (Fisher and al., Rainiko and al.) that contrast media could interfere with a red blood cell aggregation, low-osmolarity and non ionic contrast media leading to red blood cell aggregates in vitro. Clinical significance of this phenomenon is not known. We have studied 21 consecutive patients admitted in X ray department for pyelography. Sodium and meglumine ioxitalamate (Telebrix 38 R; Guerbet) osmolarity 2 100 mOsm kg--viscosity 8.5 cp at 37 degrees C is used for contrast. Red blood cell aggregation is studied by erythraggregometer Sefam. Samples are collected before (T0), 10 and 30 minutes (T10, T30) after contrast infusion. Hemogram and coagulation parameters are measured in parallel. Results show a significant increase of aggregation times at T10, with a trend towards baseline at T30. Dissociation shear rates follow the same fluctuations. We note a concomitant diminution of both fibrinogen and hematocrit levels, well-correlated with red blood cell aggregation. These results suggest a process of hemodilution which could explain the diminution of red blood cell aggregation with this type of contrast medium.