  We present first results from the MAssive Cluster Survey (MACS), a new
large-scale X-ray survey designed to find and characterize very massive,
distant clusters of galaxies. Based on X-ray detections in the ROSAT All-Sky
Survey, MACS aims to compile a sample of more than 200 X-ray luminous clusters
at z>0.3, about 50 times the number of EMSS clusters in the same redshift and
luminosity range. The MACS sample is uniquely suited to investigate cluster
evolution at redshifts and luminosities poorly sampled by all existing surveys.
  At the time of writing the MACS sample comprises 41 clusters with measured
redshifts of 0.3<z<0.56 and X-ray luminosities in excess of 6 10^44 erg/s
(0.1-2.4 keV). This early sample is thus already twice as large as the high-z,
high-luminosity subsets of the EMSS, BCS, and REFLEX cluster samples taken
together. An additional 85 MACS clusters with photometric redshifts z>0.3 are
scheduled for spectroscopic observation.
  From a preliminary analysis of a statistically well defined subsample of the
25 X-ray brightest MACS clusters we conclude tentatively that negative
evolution is not significant at the highest X-ray luminosities (Lx>10^45 erg/s)
out to redshifts of z~0.4. Our findings thus extend the no-evolution result
obtained by many serendipitous ROSAT cluster surveys at lower luminosities.
