這兩個形容詞都意為「生病的；有病的」，有時可以互換，如 His grandmother has been ill/sick for several weeks. (他祖母已經病了好幾個禮拜)。但在某些方面，它們的意思和用法卻又不同，不能混為一談，否則不鬧笑話也難。譬如說，打電話請病假是 to call in sick，不是 to call in ill，如 Sarah called in sick for two days. (莎拉打電話請了兩天病假)。
Ill 意為「生病的；有病的」，指生病的、有病的、身體不適的或身體不健康的；就此意思而言，ill 不可用於名詞之前，如 His grandpa was suddenly taken ill. (他爺爺突然病倒了)；My boss is critically ill in hospital. (我老闆在醫院裡病得很重)；Ellen was too ill to walk. (艾蘭病得連走路都不行)。
Sick 意為「生病的；有病的；反胃的；噁心的；想吐的」。在英式英語，sick (如 to be/feel sick) 專指「反胃的；噁心的；想吐的」意思，但在美式英語則指「生病的；有病的」意思；所以，John was sick yesterday. 不一定等於 John was ill yesterday. (約翰昨天病了) 的意思。但 sick 若放在名詞之前就不會引起誤解，一定表示「生病的；有病的」，如 a sick baby (一個生病的嬰兒)。例如：Ed stayed at home caring for his sick wife. (艾德待在家裡照顧她生病的太太)；The smell made us sick. (這氣味使我們想吐)；Brenda suddenly fell very sick and died a few days later. (布蘭達突然罹患重病，幾天後就過世了)。
必須注意的是，ill 若接名詞，就沒有「生病的」意思了，而是意為「壞的；有害的」，如 It is an ill wind that blows nobody any good. (凡事有利有弊)。此外，在指「病人」的集合名詞時，是用 the sick 來表示 (後接複數動詞)，而不是 the ill，如 The sick are people who are sick. (病人)。