Clinical and parasitological surveys for bancroftia filariasis were carried out in five endemic communities in north-eastern Tanzania, covering a population of 3086 individuals. High microfilarial (mf) prevalences (17.7%-34.7%) and mf geometric mean intensities (251-1122 microfilariae/ml) were observed in the communities. The mf prevalence generally increased with age, but often levelled out in the older age groups. Larger variability was observed in individual mf intensities and no clear association between mf geometric mean intensity and age or sex was seen. Hydrocele was the most common clinical manifestation (with a prevalence of 30.2%-40.0% in male subjects aged > or = 20 years) followed by leg elephantiasis (with a prevalence of 2.0%-6.8% in all subjects aged > or = 20 years). In four of the five communities, there was no significant difference in mf prevalence in males aged > or = 20 years between those with and without hydrocele. In all the communities, the mf geomtric mean intensities in microfilaraemic males with and without hydrocele were not significantly different. The present study therefore did not indicate any association between hydrocele in males (the most common type of chronic clinical manifestation seen) and presence or absence of microfilaraemia. In contrast, only two (4.4%) of the 45 subjects with leg elephantiasis were microfilaraemic. In children aged 1-15 years, mf prevalence was significantly higher among those with microfilaraemic mothers (18.0%) than among those with amicrofilaraemic mothers (7.9%). The children of microfilaraemic mothers were therefore at 2.3-fold higher risk of becoming microfilaraemic than the children of amicrofilaraemic mothers. No relationship between the mf prevalence of the children and the mf status of their fathers was observed.