OBJECTIVES: Adverse Events (AEs) has proven to be a significant cause of hospital admissions, with prevalence rate ranging from 6.5-26.1% and this constitute a significant problem with serious consequences and a challenge for public health. This intensive study is aimed at determining the prevalence of AEsas well as to assess the cause, nature, severity, preventability and outcomes in a Nigerian teaching hospital and to determine the class of suspected drugs most commonly implicated METHODS: A three months observational study of 221 consecutive adult patients of the Lagos University Teaching Hospital medical in-patient ward and out-patient clinic.  Epi-info statistical software, (Version 3.4.3, 2007) was used to analyse and determine prevalence, causality, severity and preventability. A significance level of p < 0.05 was used. RESULTS: The prevalence of AEs was 17.6% (95% CI; 12.9, 23.3) with 30 (79.5%) of these presenting at admission and 8(20.5%) developing during hospitalisation. The World Health Organization (WHO) causality assessment ranking showed that 46.2% of AEs were classified as probable, 35.9% possible and 7.9% certain.  Based on severity of the AEs, 59.0% were classified as moderate 30.8% severe, 5.1% mild and lethal respectively. 87.2% of the AEs were assessed as possibly preventable. Assessment of outcomes showed that 94.9% of the patients recovered fully while 5.1% died. The organs/systems mostly involved in the adverse events were the CNS (headache, dizziness and body pain, 22.1%), cutaneous organ (rashes and itching, 19.6%) and respiratory system (Breathlessness and cough, 13.5%) Antihypertensives (76.8%), antibiotics (10%), antiretrovirals (7.7%) and cholesterol lowering agents (7.7%) were the most commonly implicated classes of drugs. CONCLUSIONS: The study shows that AEs constitute a significant cause of hospital admissions and a good number will likely develop during hospitalization. Intensive monitoring and evaluation of AEs is feasible and will likely provide a clearer picture of clinical outcomes