The thin-needle aspiration biopsy technic was used in 469 cases to diagnose benign and malignant tumors. The success of the technic depends upon careful examination of the lesion to be biopsied and attention to the details outlined in procuring the aspiration material and preparing the smears. The most important technical consideration is creating a simulated tissue pattern in the preparation of the smears. Routine Papanicolaou, May-Grünwald-Giemsa, metachrome B, and hematoxylin and eosin stains are all useful. Special stains may provide additional evidence for specific diagnoses. Lymph nodes were the most common tissue examined by aspiration biopsy in this series. It was possible to identify the primary tumor site from the aspiration smear in most of the cases. Forty-seven breast cancers were identified from 127 aspiration biopsies of breast lumps. Breast aspirations are the most difficult to interpret, and no deviation from classic malignant smears should be attempted. The false-negative rate for breast aspirations was 7.0%. No delay in treatment occurred because of a false-negative report. Patient acceptance of the procedure was excellent.