A nonradioactive in situ hybridization technique was utilized for the rapid and early diagnosis of cytomegalovirus (CMV) infection in children undergoing kidney transplantation. The cellular samples were obtained directly from the organs thought to be affected on the basis of clinical findings: bronchoalveolar lavage during interstitial pneumonia (7 samples from 6 cases); fine-needle aspiration biopsy (FNAB) of the liver during acute hepatitis (1 case); kidney FNAB and peripheral blood where there was a > 25% creatinine rise with or without fever (26 episodes). Standard virus isolation procedures and an immunofluorescent technique on short-term cultures of human fibroblast cells were performed as a control. 6/23 children followed had a symptomatic CMV infection (4 had interstitial pneumonia; 1 had acute hepatitis, and there was 1 case of creatinine rise with fever). In all cases, the diagnosis was provided by in situ hybridization in less than 24 h. These results were confirmed 48 h later by immunofluorescence and after 5-25 days by standard viral cultures. In situ hybridization with a biotinylated probe proved to be a rapid and sensitive method for diagnosis of CMV disease, when performed on specimens obtained from the involved organs at an early stage of the infection. This diagnostic approach allowed a specific antiviral therapy to be undertaken promptly.