Anti-KIR monoclonal antibodies (mAbs) can enhance the antitumor responses of natural killer (NK) cells. We evaluated the safety of the anti-KIR2D mAb lirilumab in patients with various cancers. Thirty-seven patients with hematological malignancies ( = 22) or solid tumors ( = 15) were included in the study. Dose escalation (0.015 to 10 mg/kg) was conducted following a 3 + 3 design. Patients were scheduled to receive four cycles of treatment. In a second (extension) phase 17 patients were treated at 0.015 ( = 9) or 3 mg/kg ( = 8). No dose-limiting toxicity was recorded. The most frequent lirilumab-related adverse events were pruritus (19%), asthenia (16%), fatigue (14%), infusion-related reaction (14%), and headache (11%), mostly mild or moderate. Pharmacokinetics was dose-dependent and linear, with minimal accumulation resulting from the 4-weekly repeated administrations. Full KIR occupancy (>95%) was achieved with all dosages, and the duration of occupancy was dose-related. No significant changes were observed in the number or distribution of lymphocyte subpopulations, nor was any reduction in the distribution of KIR2D-positive NK cells. This phase 1 trial demonstrated the satisfactory safety profile of lirilumab up to doses that enable full and sustained blockade of KIR.