In 1997, a new type of histological muscle lesion, known as "macrophage myofasciitis" (MFM), was identified in pathology laboratories that were members of the Research Group on Acquired and Dysimmune Muscle Diseases (GERMMAD). In March 1998, the French Institute for Public Health Surveillance (InVS), alerted by GERMMAD, launched an epidemiological investigation with the following objectives: to precisely describe the cases identified in myopathology centers, to propose a case definition, and to generate hypotheses regarding potential risk factors for the onset of this new condition. Any individual identified in pathological anatomy centers in France in whom an image meeting the histological criteria for macrophage myofasciitis was identified was included in the study. Once the histological diagnosis of MFM was established, the myology center notified the InVS, which contacted the patient to obtain their consent to participate in the study. A standardized questionnaire was then administered. The information collected covered demographic, clinical, and biological factors; personal and family medical history, treatments and vaccinations received, and factors related to exposures—particularly occupational and environmental—were also recorded. As of May 31, 2000, 73 patients had been reported to the InVS. Of these, 53 (72%) could be located and interviewed. The report presents the results of the exploratory study and discusses hypotheses regarding potential risk factors, particularly the vaccine hypothesis. This exploratory study helped guide future actions and directions for epidemiological research. This exploratory investigation illustrates the response provided by the new health surveillance system to the emergence of a syndrome previously undescribed elsewhere and of unknown cause. This response falls within the scope of prospective surveillance missions aimed at identifying and characterizing any emerging phenomenon as early as possible, in order to facilitate its monitoring. (R.A.)