Abstract: I present a Ginzburg-Landau theory for hexagonal oscillations of the upper critical field of UPt3 near Tc. The model is based on a 2D representation for the superconducting order parameter, η = (η1 , η2), coupled to an in-plane AFM order parameter, ms. Hexagonal anisotropy of Hc2 arises from the weak in-plane anisotropy energy of the AFM state and the coupling of the superconducting order parameter to the staggered field. The model explains the important features of the observed hexagonal anisotropy [N. Keller, et al., Phys. Rev. Lett. 73, 2364 (1994).] including: (i) the small magnitude, (ii) persistence of the oscillations for T → Tc and (iii) the change in sign of the oscillations for T > T* and T < T* (the temperature at the tetracritical point). I also show that there is a low-field crossover (observable only very near Tc below which the oscillations should vanish.