An assessment of the sensitivity and reliability of renal images obtained incidentally to bone scintigraphy with 99mTc-phosphate compounds is reported, and certain patterns of abnormalities associated with those renal images are described. Renal images in 119 bone scintigrams were reviewed and correlated retrospectively with radiographic procedures done within 2 weeks of the bone images. The detection rate for renal abnormalities was 97.4%; the "false-positive" rate was 8.6% and the "false-negative" rate was 2.6%. Abnormal renal images were categorized as (A) nonvisualization of one kidney (13 cases), (B) asymmetric uptake (11 cases), (C) focal decreased uptake (4 cases), (D) bilateral decreased uptake (6 cases), (E) focal increased uptake (4 cases), And (F) miscellaneous (6 cases). The first three categories were always associated with renal abnormalities. Bilaterally decreased uptake and focal increased uptake were not reliable in detecting disease. Extensive metastatic carcinoma of the prostate was noted in three of four patients with bilateral decreased uptake and normal intravenous pyelography. We conclude that renal imaging incidental to bone scintigraphy with phosphate compounds is a valuable diagnostic test. An understanding of the different patterns of abnormalities described should improve the overall reliability of the test.