Erythema nodosum (EN) is a hypersensitivity reaction associated with many diseases. We describe a family in which 4 sisters had acute or recurrent EN. HLA typing showed a common haplotype in the affected members of the family. A review of familial EN and HLA distribution in EN is presented.Erythema nodosum (EN) is considered to be an acute hypersensitivity reaction. It may occur alone or in association with various other diseases or certain medications (1,2). There have been a few reports of familial EN, mostly occurring during tuberculosis (TB) infection (3). We recently studied a family in which all 4 sisters developed EN after pharyngitis. In 3 of them, EN was recurrent. The family, which was studied following examination of 1 of our patients (AP), who presented with acute EN, is discussed herein. Case reports. Sister 1 (AP), 41 years old, was admitted to the hospital because of fever, painful lesions in both legs, and arthralgia of the ankles and knees. A month earlier she had had pharyngitis, which was treated with amoxicillin. On examination, her temperature was 38"C, and she had several raised, red, tender nodular lesions on the pretibial region and tenderness on movement of the ankles and knees, without clear-cut synovitis. The rest of the physical examination findings were unremarkable. Laboratory studies showed an erythrocyte sedimentation rate of 45 mm/hour, and normal results of a complete blood cell count, measurements of glucose, urea, sodium, and potassium, and liver and kidney function tests. Antinuclear antibodies and rheumatoid factor were negative. The antistreptolysin 0 (ASO) titer was high (800 Todd units). A throat culture was negative, but a tuberculin test with 1/10,000 old tuberculin solution gave a strongly positive result: Radiography showed an old apical primary complex and calcified hilar adenopathy . Sputum culture was negative for Koch's bacillus.The patient was treated with naproxen, with complete resolution of her symptoms within 2 weeks. She did not recall having similar episodes in the past, but mentioned that her 3 sisters had similar symptoms on a recurrent basis.Sister 2 (SB), 54 years old, reported having had 4 episodes of EN. The first occurred when she was 26 years old. A few days after a sore throat, she noticed painful red spots on both legs and knees. She also had severe arthralgia of the ankles, knees, wrists, and shoulders. A skin biopsy confirmed the diagnosis of EN. The symptoms resolved upon treatment with aspirin. During the next 15 years, she had 3 more