Recibido el 9/Noviembre/2001. Aceptado para publicación el 12/Julio/2002. Este artículo está disponible en http://www.uady.mx/~biomedic/rb031413.pdf Artículo Original Rev Biomed 2003; 14:5-10. Artemisa medigraphic en línea 6 Revista Biomédica P Salas-Chaves, S Rodríguez-Aguilar, L Cunningham-Lucas, I Castro-Volio. SUMMARY.Utility of the alpha-fetoprotein in the prenatal diagnosis of neural tube defects and chromosomal abnormalities. Introduction. The present study focuses on the utility of AFP in prenatal diagnosis of 96 Costa Rican pregnant women. Material and methods. In these women, genetic amniocentesis and AFP levels were done between the 15 th and 25 th week of gestation. There were two main reasons for referral: abnormal ultrasound assessment and advanced maternal age. Results. The concentration values of AFP were expressed as multiples of the median, considering a value of 2.5 MoM as a normal superior limit and of 0.25 as the inferior limit. The confidence limits of the median were 562-1196 KU/l and the 2.5MoM value was located between 1404-2989 KU/l. Four cases had AFP levels higher than the median: one case of Turner Syndrome, another case of encephalocele and two normal karyotypes. Four other cases had AFP levels lower than 0.25 MoM: a case of trisomy 21, a case of trisomy 18, a case of omphalocele and a normal karyotype. Discussion. Prenatal diagnosis of fetal defects allowed genetic counseling as well as better obstetric management and pediatric care. (Rev Biomed 2003; 14:5-10)