Immediately following blood donation, the volume of circulating blood is reduced. Provided the donor eats and drinks normally, blood volume returns to normal within 24 hours. Diving within the first few hours after donating blood is not advisable because of the possibility that reduced blood flow could cause impaired inert gas washout after a dive, and thus an increased risk of decompression sickness. We have no reason to believe that it is necessary to wait more than 24 hours after donation before scuba diving.