Having shown that patients who later go on to develop PF have higher IL-10 before treatment, and higher levels of IL-6 and IL-10 during IFN-α treatment, compared with RF patients, we wanted to test whether these cytokines continue to be higher in the PF group at follow-up (six-months post-cessation of IFN-α), when the PF phenotype is established; moreover, we wanted to compare levels of cytokines also with those in patients with CFS and in controls. Here, IL6 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.