adjective

giving cause for alarm; serious.

a matter of grave concern

serious,important,all-important,profound,significant,momentous,weighty,of great consequence,vital,crucial,critical,acute,urgent,pressing,exigent,pivotal,precarious,touch-and-go,life-and-death,in the balance,dire,terrible,awful,dreadful,alarming,drastic,sore,perilous,hazardous,dangerous,threatening,menacing,risky,dicey,hairy,iffy,chancy,dodgy,egregious