The objective was to evaluate the reproductive performance of frozen sex-sorted semen (SexedUltra 4M) relative to conventional frozen semen in seasonal-calving pasture-based dairy cows. Semen from Holstein-Friesian (n = 8) and Jersey (n = 2) bulls was used. Four of the Holstein bulls were resident at or near a sex-sorting laboratory (Cogent, UK or ST Benelux, Netherlands). Ejaculates from the remaining bulls were collected in Ireland, diluted with transport medium and couriered to Cogent in temperature-controlled parcel shippers. Transit time from ejaculation to arrival at the sorting lab was 5 to 6 h. Ejaculates were split and processed to provide frozen conventional semen at 15 × 10⁶ sperm per straw (CONV) and frozen sex-sorted semen at 4 × 10⁶ sperm per straw (SS), and used for inseminating lactating dairy cows after spontaneous estrus. Pregnancy diagnosis was performed by ultrasound scanning (n = 7246 records available for analysis). Generalized linear mixed models were used to examine effects on pregnancy per artificial insemination (P/AI), with semen treatment (CONV vs. SS), bull (n = 10) and treatment by bull interaction as the fixed effects, and herd (n = 142) as a random effect. Overall, P/AI was greater (P < 0.001) for CONV than for SS (59.9% vs. 45.5%; 76.0% relative to CONV). This study was not designed to compare resident bulls vs. shipped ejaculates, but the magnitude of the difference between CONV and SS was apparently less for resident bulls (60.3% vs. 50.2%, P < 0.001; 83.3% relative to CONV) than for shipped ejaculates (58.6% vs. 40.7%, P < 0.001; 69.5% relative to CONV). The treatment x bull interaction effect was significant for shipped ejaculates (P = 0.006; range 45 to 86% P/AI relative to CONV) but not for the resident bulls (P = 0.70; range 81 to 87% P/AI relative to CONV). In conclusion, frozen SS at 4 × 10⁶ sperm per dose had lesser P/AI compared with CONV at spontaneous estrus in pasture-based dairy cows. Strategies to improve the P/AI with SS (e.g., timing of AI) require further research. Assistance from Sexing Technologies is gratefully acknowledged.