An external evaluution of the qudity of diagnosis oJ'gynecologica1 cytology specimens (Papanicolaou or "Pap" tests) in the Cytopathological Laboratory al the Mexico General Hospital was performed, with evaluation of 6,011 Pap tests randomly selected from 31,378 Pap studies previously reported as negative during 1988-1989. These cests were part of the early detectioir program for cervical cancer run by the Ministry of Health of Mexico City. A quality index jor specimens was constructed based on the presence of endocervical cells, mucus, and squamous metaplastic cells. The index of falx negatives for the diagnosis of cervical neoplasm in this sample which correspond to diagnostic error by the examiner was 0.18% (11). The constructed quality index revealed that 64. I % of specimens were low quality. The correlation between diagnostic error and low quality was .8Z Results indicate that laboratorie.r participating in early detection programs for cervical cancer in developing countries need to implement ( I ) external evaluation programs for Pap diagnosis; (2) permanent training programs in registration: and (3) monitoring promdures for collection, Jixotion, and transportation of cytologic materials.