The association of systemic lupus erythematosus (SLE) with idiopathic polymyositis or dermatomyositis is reported to occur in the range of 4-16%. Myositis can occur before or after SLE, or sporadically both diseases can be present simultaneously. This case report concerns a 36-year-old female patient suffering from Raynaud's phenomenon, polyarthralgia in the small joints of the hands, and skin changes compatible with Gotron's indications. Symmetric proximal muscle weakness of the extremities, fever of up to 40 degrees C, heliotrope rashes with erythematous changes in the face, upper arms, and posterior shoulders occurred subsequently. Laboratory analyses revealed increased acute phase reactants, hypochromic anaemia, lymphopenia, and increased levels of all muscle enzymes. Immunoserology demonstrated positive ANA, anti-Sm, and anticardiolipin antibodies (aCL), while anti dsDNA, anti Ro, anti La, and anti Jo-1 antibodies proved negative. Hypocomplementaemia and elevated levels of immune complexes were also detected. Pathologic sediment and proteinuria were revealed via urine analyses, while a kidney biopsy confirmed lupus nephritis (type IVa according to the World Health Organisation classification). Biopsy of erythematous changes of the posterior shoulder demonstrated leukocytoclastic vasculitis. Electromyography of the lower extremities established myopathic changes. Inflammation of the muscles was confirmed via magnetic resonance imaging. The patient was categorised as having two separate coexistent diseases--SLE and dermatomyositis. Both the classification criteria of the American College of Rheumatology for SLE and the diagnostic criteria for dermatomyositis, proposed by Bohon and Peter, were fulfilled simultaneously. Treatment commenced with pulses of methylprednisolone and continued with oral therapy, including Resochin. Pulses of intravenous cyclophosphamide were also administered. After six weeks of therapy, biohumoral remission of both diseases was achieved, while complete recovery from muscle weakness was accomplished after four months.