Recurrent respiratory papillomatosis (RRP) is a histologically benign disease of the larynx, trachea, and bronchi. Here we report on the histologic and molecular characteristics of 7 cases of malignant transformation of RRP to squamous cell carcinoma (SCCA). The clinical histories of 7 patients with RRP who developed SCCA were carefully reviewed. Sequential biopsies were available from 5 of the 7 cases of spontaneous transformation of RRP to SCCA and were reviewed. In addition, p53 protein overexpression and human papillomavirus (HPV) typing for all cases was examined. The average age of patients with juvenile-onset RRP was 3 years, and that of patients with adult-onset RRP was 31 years. The average age of onset of transformation to SCCA was 28 years. All patients had laryngeal involvement with RRP, and 3 of the 7 patients had tracheal extension of disease. Five patients were tracheotomy-dependent. Four of the 7 patients developed SCCA of the lung, while 3 patients developed laryngeal SCCA. There was no consistent histologic progression from squamous papilloma to papilloma with dysplasia, and all but 1 of the SCCAs were well differentiated. The overexpression of p53 protein was variable in each of the 5 patients. We detected HPV types 6/11 in papillomas from 3 patients, and HPV types 6/11, 16/18, and 31/33/51 in a papilloma of a fourth patient. No HPV DNA was detected in papillomas of 2 patients. We found HPV 6/11 in 4 of the carcinomas. We conclude that the spontaneous transformation of RRP to SCCA is not characterized by a histologic progression through dysplasia over time. Transformation can result in the loss of HPV expression. It does not appear that p53 is a molecular marker for monitoring the transformation process. Thus, these cancers may be very difficult to diagnose histologically and clinically early in the course of the transformation of the disease.