Abstract Purpose: This study examined safety, pharmacokinetics, and efficacy of veliparib, a PARP inhibitor, combined with carboplatin and etoposide in patients with extensive-stage (ED) small cell lung cancer (SCLC) and other solid tumors. Patients and Methods: The 3 + 3 design was used for dose escalation of oral veliparib in combination with carboplatin (AUC 5 on day 1) and etoposide (100 mg/m2 on days 1–3) in 21-day cycles. Veliparib dose was explored from 80 to 240 mg b.i.d. on 7-day, 14-day, or continuous schedules. Patients without disease progression continued on maintenance monotherapy (veliparib 400 mg b.i.d.) until disease progression or unacceptable toxicity. Results: Thirty-nine patients were enrolled to determine the recommended phase II dose of 240 mg veliparib for 14 days combined with carboplatin and etoposide based on long-term tolerability. Dose-limiting toxicity occurred in 1 patient (grade 2 toxic motor polyneuropathy) at veliparib 240 mg b.i.d. for 7 days. Most common adverse events related to veliparib were nausea (39%), fatigue (39%), and hematologic toxicities. Continuous dosing of veliparib 240 mg b.i.d. with carboplatin and etoposide resulted in excessive chemotherapy dose delays due to hematologic toxicity (grade 3/4 neutropenia/thrombocytopenia). Etoposide pharmacokinetics was not affected by veliparib. Confirmed responses occurred in 17 of 39 (44%) and 16 of 25 (64%) of all enrolled and ED SCLC patients, respectively. At the RP2D, confirmed responses occurred in 6 of 13 (46%) and 5 of 6 (83%) of all enrolled and ED SCLC patients, respectively. Conclusions: Veliparib (240 mg b.i.d. 14 days) plus carboplatin/etoposide can be safely combined. Phase II of this study is ongoing in first-line patients with ED SCLC.