This study evaluated data from 30 non-small cell lung cancer (NSCLC) patients to determine whether demographic, clinical, and morphologic and morphometric data that were obtained prior to treatment, could be used to predict survival. All patients had Stage III disease, and all subsequently were treated identically with concurrent radiotherapy, cisplatin, and 5-fluorouracil. The series consisted of 18 squamous carcinomas, eight adenocarcinomas, and four large cell carcinomas. Morphometric measurements of randomized selected cancer cells per case included diameter of cytoplasm, nuclei, and nucleoli. Morphologic parameters evaluated were mitotic index, histologic differentiation, and pattern of nuclear chromatin of cancer cells, and the degree of necrosis and fibrosis of tumor tissue. The lymphoid and neutrophil index defined as the ratio of lymphoid cells and neutrophils to cancer cells from randomized microscopic fields (median = 25) at 400 x magnification were also determined. Our study indicated that the peritumor lymphoid index was the only factor significantly associated with the length of survival. The correlation coefficient (Pearson r) of these two factors was 0.5 (P less than 0.005). The median survival time of patients with peritumor lymphoid index less than 3 and greater than or equal to 3 was 95 days and 376 days, respectively (Kaplan-Meier estimation). The peritumor lymphoid index was an independent prognosticator of clinical outcome of Stage III NSCLC patients, and did not correlate with any of the other parameters analyzed.