  We present spectroscopic and photometric observations of the peculiar Type II
supernova (SN) 1998A. The light curves and spectra closely resemble those of SN
1987A, suggesting that the SN 1998A progenitor exploded when it was a compact
blue supergiant. However, the comparison with SN 1987A also highlights some
important differences: SN 1998A is more luminous and the spectra show bluer
continua and larger expansion velocities at all epochs. These observational
properties indicate that the explosion of SN 1998A is more energetic than SN
1987A and more typical of SNe II. Comparing the observational data to
simulations, we deduce that the progenitor of SN 1998A was a massive star (~ 25
Mo) with a small pre-supernova radius (< 6 x 10^{12} cm). The Ba II lines,
unusually strong in SN 1987A and some faint II--P events, are almost normal in
the case of SN 1998A, indicating that the temperature plays a key role in
determining their strength.
