The objective of this study was to evaluate the clinical value of an elevated platelet count and other routine laboratory tests for predicting malignancy in patients with radiologically suspected lung cancer. Platelet count, haemoglobin, total leukocyte count, erythrocyte sedimentation rate (ESR) and serum lactate dehydrogenase (LDH) were analysed in 126 prospectively admitted patients with suspected lung cancer. The patients were divided by pathologic diagnosis into those with benign disorders (n=65) and with malignancies (n=61). Patients with lung cancer were staged (TNM) and the tumours were classified according to histological types (WHO). Thrombocytosis (platelet count >400x10(9)/l) was present in 8% (5/65) of patients with benign disease and in 57% (35/61) of patients with malignant disease (p<0.00001). The prevalence of thrombocytosis in patients with primary lung cancer was 53% (27/51). Elevated platelet count was more common in advanced disease (stage III and IV). No difference was observed between histological types. The sensitivity of thrombocytosis for predicting malignancy was 0.57 and the specificity 0.92. When elevated platelet counts, LDH and ESR were combined, a sensitivity of 0.71 and a specificity of 1.00 was achieved. The positive and negative predictive values were 1.00 and 0.89, respectively. Elevated platelet count is frequently observed in patients with lung cancer. When test results of platelet count and other routine blood analyses are combined, a high sensitivity and specificity for predicting malignancy can be achieved. These tests are clinically useful in the evaluation of patients with radiologically suspected lung cancer.