We analyze the clustering properties of a high-resolution ( \sim 10 km/s ) sample of about 1600 Lyman- \alpha lines in the spectra of 15 quasars , obtained in the framework of an ESO key-programme with the addition of literature data .
The two-point correlation function in the velocity space shows a significant signal on small velocity scales ( \Delta v \la 300 km/s ) with amplitude and significance increasing with increasing column density .
The correlation scale at z \sim 3 turns out to be 200 - 300 h _ { 50 } ^ { -1 } kpc .
A trend of increasing correlation with decreasing redshift is apparent .
The existence of over- and under-densities on scales of a few tens of Megaparsec is confirmed with a high confidence level and a number of possible structures are identified .
The present observations are found to be consistent with models of gravitationally induced correlations .
A continuity scenario between Lyman- \alpha and metal systems emerges , with a suggested physical association between the Lyman- \alpha clouds with \log N _ { HI } \ga 14 and the halos of protogalactic systems .