Research into the nature, type, and contributing factors of adverse events (AE) in primary care is required to develop successful safety interventions. This study used medical record review to determine the prevalence, preventability, severity, type, and contributory factors of AE in primary care in Madrid, Spain. The prevalence of AEs was 5%, with the majority determined to be preventable. Most resulted in mild harm, and most contributory factors were patient-related (e.g., self-administered medications).