A technique of estimating the mean hydrostatic coil pressure of the blood path (MCP) by abruptly stopping blood flow and recording the maximum outlet pressure was evaluated. It was found that this technique underestimates MCP by differing amounts depending on blood flow rate and hematocrit. Differences between estimated and measured MCP in different coil types at a variety of flow rates and hematocrits are reported. The small magnitude of the differences permits close estimates of MCP using a simple correction factor. Precise estimates of MCP, however, require measurement of hematocrit and flow rate with the technique.