Four patients with hypersensitivity reaction to carboplatin of variable severity, after previous uneventful cisplatin and carboplatin treatment, are described. Skin testing performed in two of the patients suggests a cross-reaction with cisplatin but was negative with carboplatin in one of them. The mechanism of hypersensitivity reaction to carboplatin is poorly understood and the issue of retreatment with carboplatin is controversial. Physicians should be aware of the possible hypersensitivity reaction to carboplatin and appropriate precautions should be taken.