X-ray guided localization of 215 nonpalpable, mammographically detected, breast lesions for fine-needle aspiration biopsy was performed using a two-dimensional coordinate grid. Representative material was obtained in 64% of the lesions. Very strict criteria of representativity were observed. The sensitivity of the cytologic procedure was 92%, the specificity 95%, the predictive value of positive cytologic diagnosis 88%, and the predictive value of a negative cytologic diagnosis 97%. Potential pitfalls in the localization and aspiration procedures are discussed. The results of cytologic examination should always be correlated with the mammographic findings, and further diagnostic and therapeutic decisions should be based on both modalities. In this setting, x-ray guided fine-needle aspiration cytologic study is a highly valuable diagnostic tool.