We have obtained high signal : to : noise optical spectroscopy at 5Å resolution of 27 quasars from the APM z > 4 quasar survey .
The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman- \alpha absorbers .
These data have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with \log N ( HI ) \geq 17.5 , over the redshift range 0.01 < z < 5 .
The main results are :

• Lyman limit systems : The data are well fit by a power law N ( z ) = N _ { 0 } ( 1 + z ) ^ { \gamma } for the number density per unit redshift .
For the first time intrinsic evolution is detected in the product of the absorption cross-section and comoving spatial number density for an \Omega = 1 Universe .
We find \gamma = 1.55 ( \gamma = 0.5 for no evolution ) and N _ { 0 } = 0.27 with > 99.7 % confidence limits for \gamma of 0.82 & 2.37 .

• Damped Ly \alpha systems : The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z > 3 .
Eleven candidate and three confirmed damped Ly \alpha absorption systems , have been identified in the APM QSO spectra covering the redshift range 2.8 \leq z \leq 4.4 ( 11 with z > 3.5 ) .
Combining the APM survey confirmed and candidate damped Ly \alpha absorbers with previous surveys , we find evidence for a turnover at z \sim 3 or a flattening at z \sim 2 in the cosmological mass density of neutral gas , \Omega _ { g } .

The Lyman limit survey results are published in Storrie-Lombardi , et al. , 1994 , ApJ , 427 , L13 .
Here we describe the results for the DLA population of absorbers .