A 4000 sow farm in the US using early weaning and a computerized record system was recruited. Farrowed sows were assigned into two experimental treatments: prostaglandin F2alpha injection or control. Sows were assigned by a farm worker to obtain even parity distributions between two groups in each farrowing group. A single i.m. injection of 2 ml of prostaglandin F2alpha between 24 and 48 h after farrowing was administered in the muscle immediately lateral to the vulva. Control sows received no treatment. Of 3562 farrowed sows, 1592 were administered with prostaglandin F2alpha. Parity distributions were not different between control and treatment groups. Parity was categorized into two groups: parity 1-2 or > or = 3. Mean lactation length was 18 days and there was no difference between the control and treatment groups. No main effects of prostaglandin F2alpha administration were found in either parity group on adjusted 21-day litter weight, weaning-to-first-mating interval or weaning-to-conception interval. In parity > or = 3 sows, a two-way interaction between the association of lactation length and treatment with pigs born alive at subsequent farrowing was found (P = 0.044), while no such interaction was found in parity 1-2 sows (P = 0.14). The prediction line for subsequent pigs born alive indicates that prostaglandin F2alpha administration alters the relationship between lactation length and subsequent litter size on mid- or old-parity sows.