Hoarding behaviour has been defined as the accumulation of possessions that are useless and that interfere with the ability to function. Severe hoarding may endanger not only the health and safety of the individual, but also their surrounding community. However, little information exists regarding the frequency and seriousness of this problem. This study represents the first attempt to determine the frequency of complaints about hoarding behaviour to responding health departments in one state (Massachusetts) and to explore the nature of these complaints. Eighty-eight health officers, serving a population of 1.79 million people, responded to a survey of complaints to local boards of health about hoarding behaviour. Sixty-four percent of the health officers reported at least one hoarding complaint during the five years under study; 471 case complaints were reported. In-depth information on the nature and circumstances of complaints was collected for 58 cases. Complaints typically involved multiple community agencies and occasionally resulted in significant cost to the community. Only half of the hoarders recognized the lack of sanitation in their home, and fewer than one third of complainants willingly co-operated to resolve the complaint against them. Hoarding behaviours were judged to seriously jeopardize the health of the individual and those around them. Hoarding behaviour that involved collecting animals was more serious and difficult to deal with than non-animal hoarding.