If someone or something is diagnosed as having a particular illness or problem, their illness or problem is identified. If an illness or problem is diagnosed, it is identified. The soldiers were diagnosed as having flu Susan had a mental breakdown and was diagnosed with schizophrenia In 1894 her illness was diagnosed as cancer He could diagnose an engine problem simply by listening. to find out what illness someone has, or what the cause of a fault is, after doing tests, examinations etc diagnose sb as (having) sth (diagnosis)