The seismic hazard parameter estimation using Maximum Likelihood Method (MLM) has been made in the Himalayas and adjoining regions on the basis of a procedure which utilizes mixed data containing incomplete files of large historical events (extreme part of the catalogue) and complete files of the most recent earthquakes (complete part of the catalogue) with the same threshold magnitude. The entire set of earthquake catalogues used covers the period from 1720-1990. The maximum regional magnitude, Mmax, the activity rate of seismic events, A, the mean return period, R, of earthquakes above a certain lower magnitude, M ~ m, as well as the parameter b of magnitude-frequency relationship have been determined for six seismic zones having different seismotectonic environments. Large differences of the b parameters and hazard level from seismic zone I to VI reflect the high seismotectonic complexity and crustal heterogeneity, thus suggesting that the locations of important projects like hydroelectric power plants and town development should preferably be in the zones of lower hazard level.