A noteworthy cluster of six cases of Marburg haemorrhagic fever (MHF) was identified in the Democratic Republic of Congo. One of the cases is the first infant Marburg fever patient ever documented. Three of six cases presented surprisingly mild symptoms. The results of epidemiological and virological investigations are compatible with person-to-person transmission through body fluids and with mother-to-child transmission while nurturing. The findings show that mild cases of MHF have to be expected during an outbreak and point out the difficulty to base patient management decisions on clinical case definitions alone.