Nitrite intoxication is known to be a cause of methemoglobinemia in several species. This article describes three cases of acute mortality caused by massive ingestion of nitrite by pigs raised on commercial farms. Bacterial contamination of drinking water with subsequent nitrite formation was identified as the cause of the methemoglobin formation. The present study indicates that still standing drinking water within the water systems of modern pig farms constitutes a risk for nitrite intoxication.