We studied 50 patients (28 male and 22 female) with the hyper-IgD and periodic fever syndrome. Most patients originated from Europe, namely The Netherlands (28 cases; 56%), France (10 cases, 20%), and Italy (3 cases, 6%), but 1 patient was from Japan. A hereditary component is suggested by 18 patients coming from 8 families. The syndrome is typified by a very early age at onset (median, 0.5 years) and life-long persistence of periodic fever. Characteristically, attacks occur every 4-8 weeks and continue for 3-7 days, but the individual variation is large. Attacks feature high spiking fever, preceded by chills in 76% of patients. Lymphadenopathy is commonly present (94% of patients). During attacks, 72% of patients complained of abdominal pains, 56% of vomiting, 82% of diarrhea, and 52% of headache. Joint involvement is common in the hyper-IgD syndrome with poly-arthralgia in 80% and a non-destructive arthritis, mainly of the large joints (knee and ankle), in 68% of patients. Eighty-two percent of patients reported skin lesions with some attacks; these demonstrated vasculitis histologically. Serositis has been seen in only 3 patients (6%), while amyloidosis has not been recorded in any of the patients with this syndrome. Immunizations precipitated attacks in 54% of patients. All patients had a persistently elevated serum IgD level (> 100 U/mL), and in 82% of cases the serum IgA was likewise elevated. During attacks there is an acute-phase response adjudged by leukocytosis, neutrophilia, and increased ESR. The etiology remains to be elucidated, and treatment is supportive. The hyper-IgD syndrome is distinct from other periodic fever syndromes like systemic-onset juvenile rheumatoid arthritis, adult-onset Still disease, and familial Mediterranean fever.