The nature and frequency of comorbidities upon notification of multiple sclerosis (MS) are not well known. In France, MS is one of the 30 long-term illnesses (affections de longue durée, ALD) for which 100 % of patients' health care costs are covered by the main French health insurance system. We conducted a study among 22,087 patients who had contracted MS before their 45th birthday and had obtained ALD status between 1995 and 2004. Comorbidities diagnosed at MS notification were described. The age at which MS was registered in patients with a previous comorbidity was compared with that in those patients with no previous comorbidities. Among the 22,087 patients, 21,119 (95 %) had ALD status for MS only, 653 (3 %) had a comorbidity status diagnosed at the same time as MS. Of these comorbidities, 86.8 % could be grouped into five main categories: psychiatric disease (40.2 %), autoimmune disease (24.5 %), cardiovascular disease (16.2 %), cancer (12.2 %), and metabolic disease (9.0 %). Psychiatric disorders and diabetes were more frequent in MS patients than in the general population of the same age. The mean age at request for ALD status for MS in patients with no comorbidity was 33.6 ± 7.2 years, whereas it was 36.9 ± 6.5 years in those with comorbidities. Comorbidities at MS notification are rare. Psychiatric disorders and diabetes were more frequent in MS patients than in the general population.