Background: Fibrocystic disease of the breast in one of the commonest diseases in women above 30 years of age. The assumption of it being innocuous and benign is questionable with increased incidence of malignancies developing in these women. Introduction: Understanding the pathophysiology of fibrocystic disease is essential for identifying determinants of malignant change. Case Report: A case of carcinoma of breast developing in a longstanding and recurrent fibrocystic disease is reported. Discussion: The pathological changes including the influence of hormones on the natural history of the disease arediscussed to identify the determinants of malignant transformation. Conclusion: Breast cyst fluid, patterns of cellular lining of the cysts, multiplicity, recurrence and patterns of cellular morphology are important determinants of malignant change.