The clinical trial identifier is PIONEER II. The purpose of this study is to evaluate the safety of adalimumab and to determine how well it works in the treatment of adults with moderate to severe HS. HS is a chronic skin disease that creates red, swollen, painful bumps which can break open to combine and form tunnels in the skin and scars. Sometimes these bumps can heal themselves quickly and sometimes they will become much worse and create sores that heal with multiple combined scars, or areas that do not heal. In this study, approximately 300 adults will be enrolled at treatment centers worldwide. Subject participation in this study will be up to 50 weeks. There will be a screening period, which will last from 7 to 30 days, and a study treatment period of up to 36 weeks. Study visits occur at Screening, Baseline, and Weeks 2, 4, 8, 12, 14, 16, 20, 24, 28, 32 and 36 (or sooner if subject leaves the study before Week 36). The study is divided into two treatment periods. The first period (Period A) will last 12 weeks and the second period (Period B) will last 24 weeks.