March 31 is the deadline for open enrollment of those who did not sign up for Medicare's medical insurance coverage when they were first eligible or who have dropped their enrollment.

Those who do not register by March 31 must wait until Jan. 1, 1990, to enroll, said Steve L. Jollensten, Social Security district manager.The basic premium increases 10 percent each year a person could have signed up but didn't. The basic 1989 monthly premium is $31.90.