Familial Mediterranean fever (FMF) is an autosomal recessive disorder characterized by recurrent attacks of fever, polyserositis and arthritis. A vast array of cytokines were analysed in these patients, however, little is known about the pro-inflammatory cytokine interleukin (IL)-12. Plasma IL-12 and IL-10 were measured in 24 patients with FMF (19 active, 5 inactive) and 18 healthy controls by ELISA. From 15 active patients blood was also drawn in attack-free period. Mean plasma IL-12 levels of the FMF patients (mean +/- SEM, 6.84 +/- 3.59 pg/ml) were higher than the controls (0.13 +/- 0.09 pg/ml, P < 0.001). Mean IL-12 levels of active (7.02 +/- 5.23 pg/ml) and inactive patients (6.89 +/- 5.61 pg/ml) were comparable, and they were higher compared to controls (P <= 0.001). Mean plasma IL-10 levels of the total FMF patients (3.01 +/- 1.53 pg/ml) were also higher than the controls (P=0.024). Patients had higher IL-10 levels in attacks (3.83 +/- 2.02 pg/ml) compared to levels when they were in remission (1.86 +/- 1.59 pg/ml, P=0.046). Significantly elevated IL-12 levels in FMF patients regardless of activity may suggest the presence of a pro-inflammatory state also in the inactive period of FMF. Significant increase in IL-10 levels in FMF group may point to the compensatory suppression of inflammation in active periods of the disease.