Several studies reported that Familial Mediterranean Fever (FMF) diagnosis may be missed or delayed even in countries with a high FMF prevalence. Our aim was to study on a large cohort of European FMF patients the frequency and associated factors of diagnosis delay. Clinical data were extracted from the Juvenile Inflammatory Rheumatism (JIR)-cohort. All FMF patients fulfilled Livneh Criteria and had been sequenced for MEFV exon 10. FMF-diagnostic delay (d-FMF) was defined as the duration between the onset of the symptoms and the diagnosis of more than 10 years. 960 FMF patients were enrolled: delayed diagnosis (d-FMF) was noted in 200 patients (20%). d-FMF patients were significantly older compared to non d-FMF with a median age of 46.4 years old vs. 15.5 (p < 0.0001). Women displayed more d-FMF compared to men (56 vs. 47%, p = 0.03). Clinical presentation during attacks was not statistically significant except for erysipelas-like erythema, which was higher among d-FMF patients (33 vs. 22%, p = 0.0003). The presence of one or two pathogenic MEFV mutation was not different between patients. Compared to other FMF, d-FMF patients displayed significantly more AA amyloidosis (10 vs. 2.6%, p < 0.0001) and received more biotherapy (18 vs. 3.8%, p < 0.0001). Twenty percent of FMF patients had a diagnostic delay >10 years, including more women. The differential diagnosis of abdominal attacks with menstrual pain may be an explanation, and erysipelas-like erythema may not be recognized as FMF by all practitioners.