The objective was to evaluate the accuracy of the cytologic study of fine needle aspiration biopsy and to share the authors experiences. A prospective study was done in the Hospital Regional de Especialidades No. 23, Instituto Mexicano del Seguro Social, from March 1996 to June 1997. A solid mammary nodule was diagnosed in 250 women. A fine needle punction was done in all of them. One hundred seven had cysts. The remaining 143 patients were included in this report. The clinical diagnosis was benign pathology in 87 cases and malignancy in 56. They gad a fine needle aspiration and, in those older than 35 years, a mammography was also done. The cytologic result correlated with the clinical diagnosis in 141 cases with an accuracy of 98.61%. There was any false positive result. The mammographic report correlated with the diagnosis in 30.06% of the cases, and do not correlated in 41.96%. All patients with benign pathology had a transoperative biopsy and tumorectomy. Patients with diagnosis of malignancy had a preoperative staging, which showed that 62.06% were in a surgical stage. They had a transoperative biopsy and a modified radical mastectomy. The remaining 37.94% were in a nonsurgical stage, therefore, they were referred for chemotherapy and radiotherapy. In our experience, the cytology by fine needle aspiration biopsy in solid mammary nodules is a diagnostic method with an excellent sensitivity and specificity as well as low morbility and cost.