Several studies have found a clustering of dementia in relatives of patients with frontotemporal dementia (FTD). This study analysed the familial aggregation of FTD specifically as well as the occurrence of dementia in general in first-degree relatives of patients with FTD. A family history study was carried out on 478 first-degree relatives of 74 index patients suffering from FTD. Cases of organic dementia and of FTD were diagnosed according to internationally accepted diagnostic criteria. Age- and sex-specific incidences of organic dementia and of FTD were calculated as was the proportion of FTD in relation to organic dementia in general; comparisons with clinical and population studies were made. There was a tenfold increase in the incidence of FTD in the first-degree relatives of FTD patients compared with the incidence of FTD in a population study. The proportion of FTD in relation to all types of organic dementia was much higher in relatives of FTD patients compared to the corresponding proportions in clinical and population-based studies. There was a small, non-significant difference between the present family history study and the population studies as regards the incidence of organic dementia. The findings suggest that hereditary and/or shared environmental factors are strongly involved in the aetiology of FTD. There were no indications of familial clustering of organic dementia in general in relatives of FTD patients.