Granulocytosis is common. The white count can fluctuate substan- tially and unpredictably during and between painful crises, infectious episodes, and other intercurrent illnesses. Granulocytes, platelets, and mononuclear inflammatory cells, and the inflammatory mediators that they release at the sites of vasoocclusion, are being increasingly appreciated as key contributors to the initiation and aggravation of the morbidity associated with vasoocclusive crises.