Many cats that are rendered to a shelter fail to find a new home. Older and other less desirable cats may face a significant amount of time in the shelter prior to adoption. Therefore, the present study compared the behaviour of domestic cats subjected to different lengths of time, room density (RD) and sex ratio in a shelter. Forty-six cats from a local shelter (Porto, Portugal) were included in the study. All subjects lived in group housing enclosures. Each cat's behaviour was recorded during 5min, on every other day and on three separate occasions. During each session the observer was present in the room and the following behaviours were investigated: grooming; inactivity; eating/drinking; friendly interactions; negative encounters and active behaviour. A significant greater time spent in inactivity (TSI) and time spent in negative encounters (TSNE) were found (P=0.035 and P=0.026, respectively) in cats that remained in the shelter for 7 years or longer relative to those that were housed for a shorter period of time. Time spent eating (TSE) was significantly (P=0.048) shorter in cats that spent at least 7 years compared to those that lived in the shelter for a shorter period of time. Eating frequency (EF) was significantly greater in cats sheltered for less than 1 year, relative to those in confinement for a minimum of 7 years (P=0.044). Grooming frequency (GF) and time spent grooming (TSG) were significantly greatest in cats housed in rooms with a F/M ratio of 1–2, followed by a F/M ratio greater than two and lastly a F/M ratio smaller than 1 (P=0.036 and P=0.035, respectively). TSI was shortest in rooms containing 1–2 females per male when compared to rooms with a F/M ratio of less than one (P=0.006) and larger (P=0.014) than two. TSI was significantly greater in RDs above 0.5cats/m2 (P=0.046). It is therefore possible that lengthy periods of time spent in the shelter may be associated with decreased activity levels, eating behaviour and a greater tendency towards agonistic interactions. To our knowledge this is the first study to compare the behaviour of group housed cats with the sex ratio among enclosures. Further research concerning the influence of group composition on behaviour will likely contribute to improve the welfare of cats in confinement. This study also emphasizes the need for early adoptions and improvement of the quality of life of cats that are forced to spend long periods of time in a rescue shelter.