My fiance was diagnosed with schizophrenia. how will this affect our relationship?

Depends on you: Schizophrenia is diagnosed by the presence of abnormal perceptions, thought process, and behavior which you were probably already aware of. The diagnosis does not change the person. Treatment can be very effective in decreasing symptoms and minimizing progression. If you love the person you can help them understand their illness. Go to nami.Org for resources and support groups for family members.

Hard to tell : All illness is on a spectrum. If its mild, you may be fine. If he is very affected or has severe issues, you may reconsider your relationship. Time, family history and his honesty will make a big difference. Take it one step at a time. Level of stress and job, family circumstances can make a big difference. Weigh all options before deciding. Talk to a PhD, or MD psychiatrist.