The breeding biology of Hill Mynah Gracula religiosa was studied in eight provinces of Thailand from 1991 to 1998 to evaluate how breeding behaviour has evolved to enhance reproductive success in the face of human interference. The northern race G. r. intermedia bred mostly during February to April whereas the southern race G. r. religiosa bred mostly later during April to June. Both races were cavity-nesting, non-excavating, monogamous and similar in breeding activities except for differences in body size. They nested at the bottom of deep cavities with a long entrance (mean 45 cm), in tall trees. Average nest height was 23 m. Nests were sometimes abandoned (10% of nests) during nest-building or incubation. Most Hill Mynahs (70%) had clutches of three eggs. Both parents shared incubation and feeding of young. The most detrimental factor to the reproductive success of this species was human theft of the young, rather than any natural factors. During 1991 to 1998, 80% of eggs laid were hatched, but 88% of all eggs hatched were lost before fledgling. Those illegally stolen by humans amounted to 61% of eggs hatched. Only 37 of 308 nestlings or 12% of eggs hatched survived to independence. In 1999, nests were experimentally guarded to prevent human interference, and fledgling success increased to 75% of eggs hatched. The remaining 25% of nestlings were lost to predators and unknown causes, close to the 27% lost to these causes in unguarded nests during 1991 to 1998. Although the breeding pattern of this species has evolved to promote survival of young, it does not seem to have evolved adaptations to human predation. The continuing decrease of wild Hill Mynah populations has prompted attempts at captive breeding. Data on breeding biology from this study should enhance its success.