We analyze the forward-backsvard asymmetry of the decays (π,K)ℓ +ℓ - with ℓ = μ or τ in the framework of the constrained minimal supersymmetric standard model. We find that the asymmetry is enhanced at large tan β and depends strongly on the sign of the μ parameter. For μ>0, the asymmetry is typically large and observable, whereas for μ<0 it changes the sign and is suppressed by an order of magnitude. Including cosmological constraints we find that the asymmetry has a maximal value of about 30%, produced when Higgs- and gauge-induced flavor violations are of comparable size, at a value of tan β ≃ 35. The present constraints from the B factories are too weak to constrain parameter space, and the regions excluded by them are already disfavored by at least one of BR(B → X sγ), g - 2, and/or cosmology. The size of the asymmetry is mainly determined by the flavor of the final state lepton rather than the flavor of the pseudoscalar.