  We present HI observations of 68 early-type disk galaxies from the WHISP
survey. They have morphological types between S0 and Sab and absolute B-band
magnitudes between -14 and -22. These galaxies form the massive, high
surface-brightness extreme of the disk galaxy population, few of which have
been imaged in HI before.
  The HI properties of the galaxies in our sample span a large range; the
average values of M_HI/L_B and D_HI/D_25 are comparable to the ones found in
later-type spirals, but the dispersions around the mean are larger. No
significant differences are found between the S0/S0a and the Sa/Sab galaxies.
Our early-type disk galaxies follow the same HI mass-diameter relation as
later-type spiral galaxies, but their effective HI surface densities are
slightly lower than those found in later-type systems.
  In some galaxies, distinct rings of HI emission coincide with regions of
enhanced star formation, even though the average gas densities are far below
the threshold of star formation derived by Kennicutt (1989). Apparently,
additional mechanisms, as yet unknown, regulate star formation at low surface
densities.
  Many of the galaxies in our sample have lopsided gas morphologies; in most
cases this can be linked to recent or ongoing interactions or merger events.
Asymmetries are rare in quiescent galaxies. Kinematic lopsidedness is rare,
both in interacting and isolated systems.
  In the appendix, we present an atlas of the HI observations: for all galaxies
we show HI surface density maps, global profiles, velocity fields and radial
surface density profiles.
