In vitro antigen- and mitogen-stimulated cytokine production were analysed in multiple sclerosis (MS) patients with either acute (AMS) or stable (SMS) disease and in healthy controls (HC). We also investigated whether immune responses to human endogenous retroviruses (HERV) could be detected in MS and whether these immune responses would be correlated with disease status by analysing cytokine production after stimulation of PBMC with HERV peptides. Results showed that mitogen-stimulated IL-2 and IFN-gamma was augmented and IL-10 was decreased in AMS compared to both SMS and healthy controls. Whereas the production of the metabolically active IL-12 (p70 heterodimer), was comparable in SMS, AMS and HC, production of the total IL-12 (p70 heterodimer and the p40 chain) were augmented in SMS compared to both AMS and HC. HERV-peptides IL-2 and IFN-gamma production was more frequent and more potent in AMS compared to both SMS patients and HC. HERV-specific type 2 cytokine production was more frequent and potent in SMS compared to AMS and HC. Thus a prevalent type 1 cytokine profile was seen in AMS patients, while IL-10 production predominated in SMS individuals.