Because primary breast tumors are diagnosed earlier in the clinic, procurement of sufficient amounts of tumor tissue for in-depth biological characterization is becoming increasingly difficult. We demonstrate here that relatively small numbers of tumor cells within samples of fine-needle aspirates (FNA) can be propagated in culture. Of 25 cases attempted, 12 were passageable, resulting in up to 10(7) viable cells. FNA-derived cultures were evaluated for anchorage-independence, c-erb-B2 overexpression, aneusomy, and pattern of allelic loss. In every case examined, the cultured cells closely resembled the original tumor tissue and displayed one or more tumor phenotypes. The incidence of erb-B2 overexpressing tumors was similar in passageable and nonpassageable cases (33% versus 31%, respectively). FNAs that are expanded from a wide range of clinical breast material could be useful for functional studies presently limited to rare established cell lines, such as aberrant signal transduction and gene regulation, and for testing potential anticancer vaccines and drugs.