We find that during 51 episodes of sickle cell painful crisis indirect bilirubin fell 52% from its steady state value of 2.3 +/- 1.9 mg% to a value of 1.1 +/- 0.37 mg% at the end of crisis (p less than .00000085). The indirect bilirubin decline correlates with a decrease in the dense sickle cells during crisis (r = .31, p less than .0009). During steady state, both indirect bilirubin and lactic acid dehydrogenase correlate significantly with number of dense red cells (r = .62, p less than .000002 and r = .32, p less than .02 respectively). Platelet counts, beta-thromboglobulin, Platelet Factor 4, and Fibrinopeptide A levels all were elevated during steady state and did not change during the evolution of crisis. These data demonstrate that elevated indices usually associated with platelet activation are a feature of the steady state of sickle cell disease but argue against thrombosis as a factor in the progression of a sickle cell painful crisis episode. The parallel decline of both dense cells and bilirubin during painful crises indicates that the disappearance of dense cells during crisis is not caused by hemolysis and supports the hypothesis that dense red cell sequestration, in the absence of evidence of thrombosis, is an intrinsic component of the evolution of sickle cell painful crisis.