In the minimal supersymmetric Standard Model, the heaviest CP-even Hig
gs boson H and the CP-odd Higgs scalar A are predicted to be almost de
generate in mass at the tree level, for the wide kinematic range M-A >
2M(Z) and tan beta greater than or equal to 2. However, if large soft
-CP-violating Yukawa interactions involving scalar quarks of the third
generation are present in the theory, then the CP invariance of the H
iggs potential can be maximally broken beyond the Born approximation,
and the high degree of mass degeneracy between H and A may be lifted t
hrough a sizeable HA mixing. After taking the CP-odd tadpole renormali
zation of the A boson into account, we find that the small mass differ
ence M-H - M-A, which is about 1% of the A-boson mass at the tree leve
l, can be substantially enhanced to the 25% level at the one-loop orde
r. We also find that the loop-induced mixing between the lightest CP-e
ven Higgs boson h and the A boson may be of comparable size to M-A. We
briefly discuss the main phenomenological implications of the predict
ed hA and HA mixings for the general Higgs-boson mass spectrum and for
CP-violating observables at collider and lower energies. (C) 1998 Els
evier Science B.V. All rights reserved.