The effect of varying osmolarities (0.6% to 1.5% NaCl solutions, 213 to 492 mOsm/kg H2O) on the morphology of deoxygenated sickle cells was studied quantitatively with a computer-assisted image analysis system. Discocyte-rich, less dense fractions of sickle cells (density less than or equal to 1.11) were suspended in buffered NaCl solutions (pH 7.4) of various osmolarities, deoxygenated at room temperature for up to 5 hours, and stained by Wright's solution. Microscopic images were analyzed by circular shape factor (CSF = 4 pi x [area]/[perimeter]2) and elliptical shape factor (ESF = [short axis]/[long axis]). Since these two parameters yield different values for elongated cells and for cells of other shapes, such as maple-leaf- or star-shaped cells, the morphologic changes of sickle cells can be analyzed numerically. We found that both the rate and the degree of deformation depended highly on the osmotic pressure of the media in which the cells were suspended. In hypertonic solution, most sickle cells assumed a maple-leaf shape. The deformation occurred quickly, but the degree of deformation (circular shape factor and elliptical shape factor) was lower than that found in isotonic and slightly hypotonic solutions. Although elongated cells were formed in hypotonic and isotonic solutions, deformation was slower in these solutions than in hypertonic solutions. These results indicate that the shape and the degree of deformation of deoxygenated sickle cells are highly dependent on the osmolarity of the suspending medium and that the rate of deformation is inversely related to osmolarity. The relationship between morphology of deoxygenated sickle cells and osmotic pressure of the suspending media is discussed.