We studied the breeding ecology of the White-browed Tit-Warbler (Leptopoecile sophiae) in the alpine zone of southern Tibet, elevations 4110–4780 m. The earliest breeder among local passerines, the species initiated egg-laying in early April and ended by late July. We located its domed nests in 13 species of shrubs at an average height of 0.9 m (range 0.2–2.5 m) above the ground. Clutch size averaged 4.7 (range 4–6) eggs, declining through the season. Brood size was 4.3 (range 2–6) at hatching and 3.8 (range 1–5) at fledging. Incubation lasted 20.5 (range 16–23) days, and nestlings fledged at 17.5 (range 14–21) days of age, when they were 4% above the adult weight. Of the nests we observed 66% fledged at least one young. Most pairs were monogamous, and both sexes shared all nesting duties. We noted two females attending a single nest, with a brood of normal size, and egg dumping by an additional female. At the population level, the sex ratio of offspring, determined by sexual differences in plumage of nestlings older than 6 days, did not deviate from equality. Birds breeding late in the season, however, tended to raise more females.