Sonography, disappointing as a primary screening method, has emerged as the single most helpful adjunct to mammography in evaluation of the clinically and/or mammographically abnormal breast. Sonography can reliably diagnose simple cysts presenting as palpable masses or as indeterminate, nonpalpable lesions on mammography. However, differentiation of benign from malignant solid masses cannot be reliably accomplished by sonography. The expense of an automated breast sonographic scanner has deterred many radiologists from the purchase of such a unit. The authors have used both an automated breast scanner and a real-time 10-MHz hand-held unit. This paper describes their experience with the real-time unit, demonstrating both normal and pathologic anatomy. Special emphasis has been placed on the sonographic diagnosis of a simple cyst because this lesion was the cause of one-quarter of all palpable masses and nonpalpable, mammographically dominant masses. Cysts are sharply marginated and anechoic. Posterior enhancement visible in 78 of 80 cysts was not demonstrable on all images in 25% of cysts.