Isolating an animal refers to the situation where the animal is physically fully demarcated from conspecifics without physical, visual, olfactory and auditory contact. Animals housed in separate cages in the same room are, although deprived of physical and visual contact, still in olfactory and auditory contact, and thus not totally isolated. During the fifties and sixties several studies claimed to show physiological and behavioural differences between individually and group housed rats and mice. The so-called ‘Isolation Syndrome’ characterised by changes in corticosterone levels, metabolism, growth, and behaviour was introduced, rather as a model for psychoneurosis than through any concern for animal welfare. Today, it is often stated as common knowledge in laboratory animal science textbooks that individual housing as well as isolation of rats and mice has an effect on physiology and behaviour. It is, however, unclear whether this effect actually impairs animal welfare. The aim of this paper is to analyse studies on individual housing of mice and rats to evaluate whether there is documented proof that individual housing affects welfare, and, alternatively whether it is possible to house these animals individually without negative impact on welfare, eg by providing special housing improvements. A range of studies have shown that individual housing or isolation has effects on corticosterone, the open field behaviour, barbiturate sleeping time and the metabolism of different pharmaceuticals in the animals. However, this review of 37 studies in rats and 17 studies in mice showed divergence in test results difficult to explain, as many studies lacked basal information about the study, eg information on genetic strains and housing conditions, such as bedding, enrichment and cage sizes. Furthermore, test and control groups most frequently differed in cage sizes and stocking densities, and behavioural tests differed in ways which may very well explain the differences in results. Overall, there seemed to be an effect of individual housing, although it may be small, and it seems reasonable to assume that, through making small changes in the procedures and housing environments, the effects can be minimised or even eliminated. More well-controlled and standardised studies are needed to give more specific answers to the questions this issue poses.