The reaction between the OH radical and glycolaldehyde has been theoretically studied for the first time. By means of preliminary MP2(FC)/6-31G*, B3LYP/6-31G*, CBS-Q, G2, and G3 electronic structure calculations, two main processes have been determined, CH2OHCHO + OH --> CH2OHCO + H2O and CH2OHCHO + OH --> CHOHCHO + H2O, in clear agreement with experimental data. Then the variational transition-state theory rate constants with multidimensional tunneling corrections (when necessary) (VTST-MT) have been calculated using dual-level interpolation algorithms. The theoretical rate constant for the global process at 298 K of 3.83 x 10(-11) cm(3) molecule(-1) s(-1) is in reasonable agreement with the experimental value. In the temperature range 100-350 K, we predict a clear inverse dependence of the global rate constant on temperature.