  We report on a band structure calculation and de Haas-van Alphen measurements
of KFe$_2$As$_2$. Three cylindrical Fermi surfaces are found. Effective masses
of electrons range from 6 to 18$m_e$, $m_e$ being the free electron mass.
Remarkable discrepancies between the calculated and observed Fermi surface
areas and the large mass enhancement ($\gtrsim 3$) highlight the importance of
electronic correlations in determining the electronic structures of iron
pnicitide superconductors.
