The objective of this study was to investigate to what extent fertility and lambing season of fat-tailed ewes raised under an extensive production system are related to liveweight, body condition and changes around mating. Ewe liveweight and body condition score (BSC) were recorded in June, July and August over a period of 3 years (1999–2001) in 11 flocks (eight flocks of Kurdi breed and three flocks of Kurdi × Sanjabi crossbred). Both Kurdi and Sanjabi are native fat-tailed breeds in western Iran. In total, 3278 lambing records relating to 1592 ewes and data on mating date, liveweight and BCS of 1930 ewes (3975 records) were used for statistical analyses. The statistical model for bodyweight and body condition data included fixed effects of breed group, flock nested within breed group, year, the future lambing status following summer breeding (barren, autumn lambing and winter lambing), ewe age and all two-way interactions. Breed group had a significant effect on ewe liveweight (P < 0.01). However, there was no significant difference between BCS of the two breed groups around mating. Ewe liveweights measured in June, July and August had significant effects on fertility and lambing season (P < 0.01). Ewes that produced lambs were generally heavier at mating than barren ewes (P < 0.01). Autumn-lambing ewes were heavier than winter-lambing ewes in June and July (P < 0.01). Effects of changes in liveweight on fertility and lambing season were significant (P < 0.01). The effects of BCS in June, July and August (P < 0.01) and changes in BCS during the mating period (P < 0.05) on fertility and lambing season were also significant. The mean BCS (in June and July) of autumn-lambing ewes was greater than that of the winter-lambing and barren ewes (P < 0.05). The proportion of autumn-lambing ewes significantly increased as BCS in July increased, whereas the proportion of barren ewes decreased (P < 0.01). Fertility was associated with age so that as the age of the ewes increased from 2 to 7 years, the proportion of barren ewes significantly decreased from 29 to 5% (P < 0.01). There was a relatively high proportion of very thin ewes of 2 and 3 years of age. These findings imply that low body condition is a particular problem in younger ewes suggesting the importance of giving special nutritional treatment to younger ewes to reduce the proportion of barren ewes.