Background: Affective flattening has been described as enduring, but long term follow-up studies of first episode psychosis patients are lacking. Objective: The aim of this study was to follow the symptom development of flat affect (FA), over a 10. year follow-up period, with focus on prevalence, predictors and outcome factors including social functioning. Methods: Three-hundred-and-one patients with FEP were included at baseline, 186 participated in the 10. year follow-up. These were followed on PANSS item N1 (FA) from baseline through 5 follow-up assessments over 10. years. Patients were grouped as having never-present, improving, deteriorating, fluctuating or enduring FA. The groups were compared on baseline variables, variables at 10. year follow-up, and social functioning throughout the follow-up period. Results: Twenty nine percent never displayed FA, 66% had improving, deteriorating or fluctuating FA, while 5% of patients had enduring FA. Premorbid social function predicted enduring FA. The patients with enduring, fluctuating and deteriorating FA did poorer on all outcome variables, including remission and recovery rates. The enduring FA group did significantly poorer in social functioning over the 10. year period. Conclusions: FA is expressed at some point of time in the majority of FEP patients in a 10. year follow-up period, and appears more fluctuant than expected from the relevant literature. FA is associated with poorer outcome after 10. years, and enduring FA to poorer social function at all points of assessment.