The phenotypic screening for genetic haemochromatosis (GH) relies upon the determination of transferrin saturation (TS). In large-scale screening programs, the time of blood sampling can be uneasy to control. We studied the circadian variations of TS at 08.00 hours, 12.00 hours, 18.00 hours and 00.00 hours in 46 C282Y homozygous patients (GH) and 47 non-GH patients (NH), to determine whether the time of blood sampling influenced the results of screening. In both groups, there were significant circadian variations in TS, with the highest values at 08.00 hours and the lowest at 00.00 hours. For any given time-point, TS was significantly higher in the GH group when compared with the NH group (P < 0.0001). For both groups, there was a significant decrease in TS between 08.00 hours and 00.00 hours (P < 0.0001) but this decrease was not as significant in GH when compared with NH patients (interaction P < 0.0073). Receiver operating characteristics (ROC) curves generated for TS at 08.00 hours, 12.00 hours, 18.00 hours and 00.00 hours, presented the same efficiency of diagnosis of GH, with TS threshold varying between 64% at 08.00 hours and 36% at 00.00 hours. In conclusion, for screening studies of C282Y homozygosity, determination of transferrin saturation may be performed at any time during the day.