Mirella R. Cimini, 18, who was a 1979 graduate of Potamac Senior High School, died Saturday at the National Institutes of Health, where she was being treated for cancer.

She was named an "outstanding Young American High School Student" by the Rotary Club of Southern Prince George's County and was given an American Legion Medal for "courage, honor and leadership."

Miss Cimini was a member of the Holy Family Catholic Church in Hillcrest Heights, where she had done volunteer work with children.

She was blessed by Pope John Paul II at Trinity College during the pontiff's recent visit to this country. On the day before her death, Miss Cimini received a letter from an official of the Veteran's secretarial of state conveying to her the Apostolic blessing. It said, "His Holiness wishes me to assure you that he will pray for you."

Miss Cimini was a native of Washington and a resident of Cheltenham, Md. She was a graduate of Holy Family Catholic School in Hillcrest Heights.

Survivors include her parents, George and Anna Cimini, and two brothers, Angelo and Roberto, all of Cheltenham; and grandparents, Angelo and Cesira Cimini, both of Rome, Italy and Maria and Attilio Santi, both of Marlow Heights, Md.