The Baseline phase will begin with the Screening visit (day -21 = Visit 1) and continue for three weeks; DHAFs (Daily Health Assessment Forms) will be collected from all subjects from Day - 21 to Day 0. DHAF information including MWAS (Muckle-Wells Activity Score), or SCHAS (Schnitzler Activity Score) values from this period will be used for the baseline phase evaluation. Inclusion to receive rilonacept will occur on day 0 (= Visit 2).