The Achi community of south-east Nigeria was given mass ivermectin therapy to control endemic onchocerciasis. 7556 subjects (75.6% of those eligible) were dosed. 992 patients (13.1%) complained of adverse effects, mostly within one week of dosing. Adverse events were mainly of the Mazzotti type. Exacerbation of pruritus (71.2%), oedema (47.4%), headache (46.4%), and worsening of rash (24.4%) were the most common. In 962 subjects (97%), adverse events were mild and did not prevent work. Two patients suffered severe sustained postural hypotension. The incidence of adverse events was greater in villages with a high load of microfilarial infection.