Purpose: This study investigated whether sequential administration of erlotinib and chemotherapy improves clinical outcomes versus chemotherapy alone in unselected, chemotherapy-naïve patients with advanced non-small-cell lung cancer (NSCLC). Patients and Methods: Previously untreated patients (n = 154) with stage IIIB or IV NSCLC and Eastern Cooperative Oncology Group performance status of 0 or 1 were randomly assigned to receive erlotinib (150 mg/d) or placebo on days 15 to 28 of a 4-week cycle that included gemcitabine (1,250 mg/m2 days 1 and 8) and either cisplatin (75 mg/m2 day 1) or carboplatin (5 = area under the serum concentration-time curve, day 1). The primary end point was nonprogression rate (NPR) at 8 weeks. Secondary end points included tumor response rate, NPR at 16 weeks, duration of response, progression-free survival (PFS), overall survival (OS), and safety. Results: The NPR at 8 weeks was 80.3% in the gemcitabine plus cisplatin or carboplatin (GC) -erlotinib arm (n = 76) and 76.9% in the GC-placebo arm (n = 78). At 16 weeks, the NPR was 64.5% for GC-erlotinib versus 53.8% for GC-placebo. The response rate was 35.5% for GC-erlotinib versus 24.4% for GC-placebo. PFS was significantly longer with GC-erlotinib than with GC-placebo (adjusted hazard ratio, 0.47; log-rank P = .0002; median, 29.4 v 23.4 weeks); this benefit was consistent across all clinical subgroups. There was no significant difference in OS. The addition of erlotinib to chemotherapy was well tolerated, with no increase in hematologic toxicity, and no treatment-related interstitial lung disease. Conclusion: Sequential administration of erlotinib following gemcitabine/platinum chemotherapy led to a significant improvement in PFS. This treatment approach warrants further investigation in a phase III study.