The objective of this study was to assess the accuracy of estimating the size and number of corpora lutea (CL) and ovarian follicles by ultrasonography (US) of the ewe in standing position. US observations were compared with those made at subsequent postmortem examination of the ovaries. Corriedale ewes (n = 50) of unknown reproductive history and at random stages of the oestrus cycle were used for the study. Transrectal US was performed using a 7.5 MHz transducer with the ewe in standing position. The ewes were slaughtered 12 h after the US examination and the ovaries collected, dissected and the number and size of the CL and follicles evaluated. CL were classified as functional or non-functional on the basis of their colour and follicles were classified by size (2, 3, 4 and greater than or equal to5 mm). Accuracy of US was assessed by calculating its positive predictive value and sensitivity. The data were evaluated by Pearson correlation and linear regression analysis. The predictive value and sensitivity of US was 100% for the presence and 96% for the absence of CL. In four out of five ewes with double ovulations, the presence of both CL was correctly diagnosed by US. For functional CL, the sensitivity of US was 100%. The regression coefficient for CL diameter was significant (P less than or equal to 0.001; r(2) = 0.4; n = 35). The size of 85 of 117 follicles was accurately determined. The correlation between numbers of follicles counted by US and postmortem increased with size of follicle from r = 0.44 (P <0.01) for 2mm to r = 0.85 (P <0.001) for :5 mm diameter follicles. The regression between the diameter of follicles determined by the two methods was significant (P <0.001; r(2) = 0.8; n = 117). The predictive value of US for the number of follicles was high (98-100%) for all follicle sizes except for 3 turn diameter follicles (predictive value 71%). Similarly, the sensitivity was high for all sizes of follicles (90-95%) except for those of 2 mm diameter (62%). It was concluded that ultrasound scanning provides a highly accurate method for determining the number of CL and follicles greater than or equal to4mm diameter but that its predictive value and sensitivity are lower for smaller diameter follicles. The regression equation for diameter measured by US on that evaluated postmortem explained more of the variation for follicles than for CL. (C) 2003 Elsevier B.V. All rights reserved.