咳血又称为咯血，是一种症状，指肺或气管中的出血，经由咳嗽而吐出。多半是因为所吐出的痰中带有血丝而被发现。最常见的原因就是支气管炎、肺炎、肺肿瘤、肺结核等疾病。或是使用抗凝血药物。若是发生在儿童，则较可能是因为异物进入呼吸道所引起。长年抽烟的人也常见到痰中带血。在中国过去提到肺痨，也常会描述症状为痰中带血。此外严重受伤也会导致咳血。

咳血在临床上需要和呕血（上消化道出血）相鉴别。可借由观察血液颜色也能初步分别出血的可能来源，例如，咳出鲜红、带些气泡的血液可能来自呼吸道；暗红、咖啡色则可能是消化道出血。

Hemoptysis in American English, spelled haemoptysis in British English, is the act of coughing up blood or blood-stained mucus from the bronchi, larynx, trachea, or lungs. This can occur with lung cancer, infections such as tuberculosis, bronchitis, or pneumonia, and certain cardiovascular conditions. Hemoptysis is considered massive if there is more than 300 mL (11 imp fl oz; 10 US fl oz) of blood lost in 24 hours. In such cases, the primary danger comes from choking, rather than blood loss.