The historical background to the disorder is unfolded. It was originally regarded as a dermatological curiosity, and later sarcoidosis was recognized as a disorder diffusely involving most tissues of the body. Clinical syndromes described include those presenting to the dermatologist, ophthalmologist, chest physician and radiologist, gastroenterologist and neurologist. Tissues commonly involved are lungs, lymph nodes, eyes, skin and bone in that order of frequency. Granulomatous uveitis in the presence of skin lesions should always arouse the suspicion of sarcoidosis; ocular and bone involvement are frequently associated with lupus pernio, whereas lymphadenopathy and splenomegaly are more commonly linked with plaques and maculo-papular eruptions. Bone cysts are rare in the absence of skin lesions, so routine radiography of hands and feet is of little diagnostic value. The basic criteria for establishing the diagnosis of sarcoidosis are twofold: (a) Suggestive clinical and/or radiological features with evidence of generalized involvement. (b) Histological proof of sarcoid tissue from at least one tissue. Evidence of one without the other is insufficient, for clinical or radiological manifestations alone present too wide a differential diagnostic problem and, conversely, isolated histological evidence of sarcoid tissue could be construed as a local sarcoid-tissue reaction. Histological confirmation is obtained whenever possible from accessible involved tissues (skin or lymph node). Otherwise blind biopsy of liver, scalene lymph node or gastrocnemius muscle is employed, or alternatively the Siltzbach-Kveim test is performed. It is a safe, simple and specific outpatient skin test, which provides histological confirmation in three-quarters of patients with sarcoidosis. The natural history of sarcoidosis and the response to treatment vary with the type of disease--namely subacute (transient) or chronic (persistent) forms of sarcoidosis. The only treatment which favourably influences clinical, or radiological or histological features of the disease is corticosteroid therapy. Indications for treatment are: ophthalmic involvement, steadily worsening chest radiograph, breathlessness, persistent hypercalciuria, disfiguring skin lesions, neurological involvement, disordered glandular function.