The coexistence of an epidemiological register and a multidisciplinary centre for prenatal diagnosis promoted us to report data collected during six years (1990-1995) in Isère county on prenatally detected chromosomal aberrations. During the whole study period prenatal diagnosis strategy towards chromosome aberrations was based solely on maternal age and ultrasound examination. Results showed a respective contribution of one-third/two-thirds for the two detection modes (maternal age/ultrasound signs). From 1990 to 1995 a significant increase in the proportion of prenatally detected autosomal aneuploidy was observed, from 52 per cent to 75 per cent (P < 0.001). This significant variation was mainly due to an increase in the proportion of prenatally detected trisomy 21 cases, and to an increase in the proportion of aberrations which were detected through first trimester ultrasound examination. The highest positive predictive values were observed for polymalformation, cardiac anomalies and cystic hygroma ultrasound signs (51 per cent, 21 per cent and 26 per cent, respectively). Our results for trisomy 21 are close to those obtained in other studies, even when prenatal strategies are different. Their interest lies in the fact that they can be considered as a reference level of prenatal diagnosis efficiency due to a strategy based on maternal age and ultrasound signs, a level which has to be taken into account when evaluating the benefits of additional serum screening policies in other studies.