The western lowland gorilla (Gorilla gorilla gorilla) is one of the four subspecies of gorillas, but the only one which is breeding successfully in captivity. Details of the breeding results have been published in the International Studbook for the Western Lowland Gorilla. These results show a birth sex-ratio of 0,50, which means an equal number of male and female offspring is born. The overall birth sex-ratio of 0,50 does not differ from gorillas in the wild. To simulate the wild situation, gorillas are housed in family groups with one adult silverback male, a number of adult females and their offspring. But because there are approximately equal numbers of male and female gorillas born in captivity, housing gorillas in social breeding group inevitably means that there are more males than available breeding situations. Housing the surplus males in all-male groups is only a short-term solution. It would be better to avoid the male problem by controlling the sex-ratio of the offspring. The breeding programme could then be restructured to produce predominantly female offspring. The objective of this research is to gain insight in the birth sex-ratio in the captive western lowland gorilla population.