Large cell lymphomas often challenge the diagnostic flow cytometrist. The purposes of this study were to improve our protocols for diagnosing large cell lymphomas and to correlate flow cytometric (FC) data with demographic and histologic features. We identified 63 cases of large B-cell lymphoma between January 1, 1995, and July 30, 1999, and reviewed the diagnostic slides and FC light scatter and staining patterns. The 51 lymphomas with adequate material for systemic review fell into 2 light scatter patterns: "clear cut," with large abnormal cells (high forward scatter relative to normal lymphocytes), 17 cases (33%); and "complex," 34 cases (67%). Clear-cut cases were more mitotically active (average of 42 vs 25 per 10 high-power fields), with higher cellularity. Apoptosis, geographic necrosis, and sclerosis were present histologically in many cases, regardless of FC findings. We conclude that morphologic features of large cell lymphomas do not predict which cases will be difficult to diagnose by FC. Gating strategies can be critical to improve the diagnostic yield.