On January 31 1854, Jules Baillarger delivered a lecture to the French Imperial Academy of Medicine describing a new variety of insanity, "la folie à double forme", whose main feature was the occurrence of both manic and depressive episodes in the same patient. Immediately his colleague Jean-Pierre Falret pointed out that he had already published a description of the same disorder, that he had named "folie circulaire" and, on February 14, presented a lecture on the subject to the Academy. Baillarger accused him of plagiarism, contending that Falret had used the two weeks between the two lectures to attribute the main characteristics of la folie à double forme to his folie circulaire, whose description now bore little relation to the (very short) previous report that Falret had evoked as proof of his priority. Falret subsequently showed great restraint in the controversy, while Baillarger reiterated and extended his accusations until his death. An objective study of the printed material not only establishes Falret's clear priority, but also demonstrates that Baillarger's accusations of plagiarism are unfounded, as the descriptions of la folie circulaire and la folie à double forme differ on many important points, the first being much closer than the second to our present conceptions. Falret's discovery is thus a landmark in the nosology of mental disorders. Contrary to the then-widespread belief in the existence of a single entity--" mental alienation"--with several symptomatic manifestations, Falret affirmed that, in psychiatry as in the rest of medicine, separate disease entities existed. While their etiology was still unknown, they could be distinguished by their syndromic expressions and their outcome; on this basis, Falret considered that la folie circulaire was, together with general paralysis, the only true mental disorder so far identified. His principles were subsequently adopted by Kraepelin, whose main nosological concepts, established around 1900, are still in use today. Kraepelin attributed most psychotic manifestations to two diseases, namely dementia praecox (later renamed schizophrenia) and manic-depressive psychosis; the two differed mainly by the progression of the former to a final state of mental deterioration. Falret's folie circulaire was incorporated in the second disease, as one of its many symptomatic forms. It regained its autonomy only in 1966 when Angst and Perris demonstrated the specificity of its heredity and named it bipolar disorder. Recent studies suggest that the boundaries of this disorder extend far beyond those of its classical description. The so-called bipolar spectrum now includes, on the basis of clinical, biological, genetic and therapeutic arguments, the manic and hypomanic, and even purely depressive manifestations of mood disorder. One hundred and fifty years after its inception, Falret's concept of folie circulaire has become one of the main focuses of psychiatric research.