This randomized, multicenter, double-blind, placebo-controlled, parallel-group study will assess the efficacy and safety of lebrikizumab in patients with asthma whose disease remains uncontrolled despite daily therapy with an inhaled corticosteroid and a second controller medication. Patients will be randomized in a 1:1:1:1 ratio to receive double-blind treatment with subcutaneous lebrikizumab ("highest", "middle", "lowest" dose) or placebo every 4 weeks for 52 weeks, in addition to their standard-of-care therapy. This will be followed by a 52-week double-blind active treatment extension. The anticipated time on study treatment is up to 104 weeks. There will be a safety follow-up of 24 weeks after the last dose of study drug for all patients.