Two of the many factors which may affect the accuracy of pregnancy diagnosis using milk progesterone levels are day of sampling and number of samples taken per cow. These two aspects were analysed using information obtained from progesterone profiles encompassing 359 pregnancy tests. Where a single sample was obtained between 20 and 24 days after insemination the accuracy of pregnancy diagnosis was less than for two samples taken on the 21st and 22nd day (89,1 vs 97,4% accuracy). The best time to take a single sample was 24 days after insemination when only 1,5% incorrect classifications were made.