Do type Ia supernovae prove Λ>0?

The evidence for positive cosmological constant Lambda from Type Ia supernovae is re-examined. Both high redshift supernova teams are found to underestimate the effects of host galaxy extinction. The evidence for an absolute magnitude-decay time relation is much weakened if supernovae not observed before maximum light are excluded. Inclusion of such objects artificially suppresses the scatter about the mean relation. With a consistent treatment of host galaxy extinction and elimination of supernovae not observed before maximum, the evidence for a positive lambda is not very significant (3-4sigma ). A factor which may contribute to apparent faintness of high-z supernovae is evolution of the host galaxy extinction with z . The Hubble diagram using all high-z distance estimates, including SZ clusters and gravitational lens time-delay estimates, does not appear inconsistent with an Omega(o) =1 model. Although a positive Lambda can provide an (albeit physically unmotivated) resolution of the low curvature implied by cosmic microwave background (CMB) experiments and evidence that Omega(o) <1 from large-scale structure, the direct evidence from Type Ia supernovae seems at present to be inconclusive.