Bilateral bone-marrow examinations from the posterior iliac crests were routinely performed as a pretreatment staging procedure in 89 consecutive patients with small-cell anaplastic carcinoma of the lung. Bone-marrow involvement was found in 20 patients (22.5%), in 9 (10.1%) only on one side, and in 11 (12.4%) on both sides. Aspiration was found to be significantly superior to biopsy, being positive in 25 examinations as compared with 14 positive biopsies. Compared with unilateral bone-marrow examination the positive findings increased by approximately 30%. Bilateral bone-marrow examinations are recommended in patients in whom the detection of bone-marrow metastases will have therapeutic implications.