What can long-baseline neutrino oscillation experiments tell us about neutrino dark matter?

We define the ratio R = (No. of observed electron events)/(No. of expected muon events - No. of observed muon events) as a measure for the relative importance of nu(mu) --> nu(e) and nu(mu) --> nu(tau) oscillations which will be measurable in coming long-baseline neutrino experiments. We then argue that if neutrino oscillation is observed with 0.02 < R < 0.87 it implies the rejection of the neutrino dark matter hypothesis in a wide range of the mixing parameters which is consistent with the atmospheric neutrino anomaly. Our argument is valid if neutrinos come with only three flavors without steriles and if they have a hierarchical mass pattern. (C) 1998 Elsevier Science B.V.