B3LYP/6-31G(d) calculations have been performed on the ring opening of cyclopropylcarbinyl radical 1 to 3-buten-1-yl radical 2. The dynamics of the reaction have been computed with canonical variational transition state theory (CVT), both with and without inclusion of small-curvature tunneling (SCT). The CVT + SCT calculations predict that 1 should undergo rapid and temperature-independent ring opening to 2 at cryogenic temperatures, by tunneling from the lowest vibrational level of 1.