  It is becoming clear that luminous extragalactic X-ray and sub-mm sources are
essentially distinct populations. Thus, if high redshift sub-mm sources
represent massive spheroids in formation, there must be a time lag between the
major epoch of star-formation and the appearance of a visible quasar. Despite
this distinction, I find tentative evidence for a puzzling angular
cross-correlation between X-ray sources and bright sub-mm sources in two
independent fields. If this signal is due to large-scale structure it would
argue for a low redshift (z<2) for many of the SCUBA sources. Alternatively, I
suggest that the effect may be enhanced by gravitational lensing. The
exceptionally steep slope of the bright sub-mm counts makes this population
particularly prone to lensing bias. An apparent correlation may therefore be
produced if X-ray sources trace the intervening large scale structure.
