Previous studies have demonstrated circadian rhythms in the onset of thrombotic events, which occur most commonly in the morning, and also in the fibrinolytic activity of the blood which has a peak in the evening and a trough in the morning. There has recently been increasing interest in the role of white blood cells (WBCs) and free radicals (FRs) in thrombosis. No one has yet investigated the potential circadian variation of WBC aggregation and FRs in humans. We studied the circadian rhythm of WBC aggregation and FR status in 10 healthy male volunteers. Six blood samples were collected at 4 hourly intervals from 12:00 (mid-day) until 08:00 the following morning. The volunteers carried out normal daily activities until 00:00 at which time they went to bed and they remained in bed until 08:00. The following were measured on each sample: WBC aggregation; thiobarbituric acid reactive substances (TBARs), plasma thiols (PSH), red cell lysate thiols (LSH), glutathione (GSH) and superoxide dismutase (SOD) which are all altered in the presence of FR activity. The following parameters demonstrated significant circadian rhythms, WBC aggregation p less than 0.001, TBARs p less than 0.015, PSH p less than 0.001, LSH p less than 0.002. WBC aggregation was lowest at 09:00 and highest at 00:00-04:00. TBARs and PSH both had a peak at 16:00 and a trough at 04:00. LSH had a peak at 12:00 and a trough at 08:00. As the behaviour of WBCs and FR status influence the flow properties of blood, a circadian rhythm in WBC function and FR status may contribute to the time of onset of thrombotic diseases. Moreover, with many studies being currently undertaken in this area, our work indicates the need to standardize sample times.