In 1959 a nervous disorder developed in his right hand but he continued playing repertoire for the left hand. He arranged more than 80 pieces for the left hand; his Apparitions, voor piano (linkerhand alleen) (1961) were described by one reviewer as "intense and expressive piano music that should make it a joy for a pianist to develop his left hand." Slowly he regained power over his right hand. He became musical director of the Dutch Broadcasting Foundation and promoted Dutch contemporary music. He made many recordings but also continued composing. His compositions include music for piano solo such as Variations imaginaires (1967); orchestral music; vocal work; and chamber music such as Sonatine pastorale for oboe and piano (1961). It has been stated that Dutch composer Gerard Schurmann composed his Bagatelles (1945) for de Groot, but this is not, in fact, the case. He died in Amsterdam in 1993.