Ivermectin treatment may induce marked adverse effects in those harbouring > 8000 Loa microfilariae (mff)/ml of blood, individuals with > 30 000 Loa mff/ml being at risk of developing serious neurological reactions. It is thus necessary to delineate the geographical areas where such responses may occur. To determine if the simple measure of prevalence of Loa microfilaraemia would be appropriate to identify the communities at risk, the relationships between prevalence and intensity of Loa microfilaraemia were investigated in 67 villages in Cameroon. The prevalence recorded in the adult population was found to be closely related to each of the indicators of infection intensity investigated. For example, when the prevalences of Loa microfilaraemia in adults were 20%, 30% and 40%, approximately 5%, 9% and 16% of the adults had microfilarial loads exceeding 8000 mff/ml, respectively; the corresponding percentages of adults with > 30 000 mff/ml were about 1%, 3% and 5%-6%. Thus it seems that, in areas where loiasis is co-endemic, the monitoring procedure during large-scale ivermectin treatments for the control of onchocerciasis only needs to be strengthened in those communities where the prevalence of Loa microfilaraemia in adults exceeds 20%.