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Repetitive manipulation of thumbs and fingers too much can cause de Quervain tenosynovitis. Some smartphones are becoming larger than their predecessors, with larger display, which might lead to more occurrence of tenosynovitis. A Japanese man in his 40s developed de Quervain tenosynovitis after buying a smartphone while using his thumb to handle it. Ceasing to use the thumb and using his right index finger while holding the phone eased his symptoms and he recovered without specific treatment. Health care providers should be aware of this phenomenon, particularly related to newly purchased smartphones.