Whole blood viscosity (WBV) was measured on six healthy horses and ten horses presented for colic surgery. A Wells-Brookfield cone-and-plate microviscometer at 6 rpm was used to determine WBV and WBV was adjusted for packed cell volume (PCV). The ten colic horses were divided into two groups: five horses that died or were euthanatized (group 1) and five horses that were discharged after surgery and medical therapy (group 2). The mean WBV for each group was compared using analysis of variance followed by Duncan multiple-range tests. Mean WBV of group 2 (5.81 +/- 1.48 centipoise [cp]) and normal horses (7.1 +/- 2.3 cp) was significantly (P less than 0.01) lower than group 1 (17.15 +/- 5.71 cp). This preliminary trial shows that horses that died or were euthanatized had a significantly higher WBV than horses surviving. Since viscosity was adjusted for PCV, the differences may represent changes in other factors such as fibrinogen, rouleaux formation, red blood cell deformability, PaO2, or intracellular calcium ion concentration. Thus, increases in WBV in horses with colic may indicate severely compromised regional perfusion that may lead to significant tissue damage. In this preliminary study, it appears that WBV may be a prognostic aid in horses with colic.