  Using the IRAM 30 m telescope, a mapping survey in optically thick and thin
lines was performed towards 46 high mass star-forming regions. The sample
includes UC H{\sc ii} precursors and UC H{\sc ii} regions. Seventeen sources
are found to show "blue profiles", the expected signature of collapsing cores.
The excess of sources with blue over red profiles ([$N_{\rm blue}$ -- $N_{\rm
red}$]/$N_{\rm total}$) is 29% in the HCO$^+$ $J$=1--0 line, with a probability
of 0.6% that this is caused by random fluctuations. UC H{\sc ii} regions show a
higher excess (58%) than UC H{\sc ii} precursors (17%), indicating that
material is still accreted after the onset of the UC H{\sc ii} phase. Similar
differences in the excess of blue profiles as a function of evolutionary state
are not observed in low mass star-forming regions. Thus, if confirmed for high
mass star-forming sites, this would point at a fundamental difference between
low- and high-mass star formation. Possible explanations are inadequate
thermalization, stronger influence of outflows in massive early cores, larger
gas reserves around massive stellar objects or different trigger mechanisms
between low- and high- mass star formation.
