Nursing behaviour tends to be synchronized in lactating sows. However, the nursing synchrony has not been quantified in the sows housed in commercial conditions. In this study, the amount of nursing synchronization was quantified, the dependence of the degree of synchronization on the spatial distance between sows was examined, and the effect of nursing vocalization playback on nursing frequency and nursing synchronization was assessed in a large commercial pig farm. Sows were housed in individual farrowing crates, 14 sows per room. Twelve rooms were assigned to one of four treatment combinations, which was age of piglets (6-10 days vs. 13-17 days), and bass frequency on the loudspeakers (bass on vs bass off). The playback of nursing vocalization was employed for three minutes every 45 min, 55 min, or no playback at all for three days. The nursing behaviour and its synchronization were recorded in two ways: through a 2 h direct interval sampling observations of all sows and through a 6 h video recording of three focal sows. The nursing synchronization was higher than the random frequency. Synchronization in pairs of sows declined with increasing distance. The playback of nursing sounds did not increase either the nursing frequency or the nursing synchronization. It is concluded that nursing synchronization is high in large rooms with crated sows, that is not a sole consequence of synchronization in general activity. Nursing synchronization depends on the distance between the sows, and an increase in nursing frequency is not always reliably induced by a playback of pig nursing vocalizations.