The frequency and severity of the painful sickle cell crisis vary greatly among affected patients. Aside from a high level of Hb F(greater than 20%) there is no established parameter which may modulate the clinical severity of the disease. In this paper we describe two groups of adult patients with homozygous SS and present their characteristics. The division into these two groups was on the basis of relatively low RBC deformability (less than or equal to 37% of control) and high RBC deformability (greater than 65% of control) in the steady state. None of the patients had alpha-gene deletion and all had Hb F level less than 6.0%. Each patient was followed for a minimum of 3 years. The number of dense cells was quantitated by centrifugation on discontinuous Stractan gradient. RBC deformability index in isotonic medium (DI 290) was determined by ektacytometry and expressed as % of control. The patients with low RBC deformability had significantly less painful crises and more leg ulcers than those patients with high RBC deformability. The average number of dense cells was 22.2% and 9.8% of total circulating cells in the first and second group respectively. Moreover, the group with high red cell deformability had 33% mortality during the study period whereas no deaths occurred in the group with low RBC deformability. The data indicate that there is a subset of patients with SS who have relatively few painful crises despite low Hb F level. We wish to designate these by the acronym MIDDD syndrome: Mild disease as far as painful crises are concerned, increased number of Dense cells, and Decreased red cell Deformability. In addition these patients have high incidence of leg ulcers, have low incidence of urinary tract infection, and less mortality. Cellular factors seem to contribute to the incidence of painful crises.