Objectives: Exposure and clinical data concerning cases of toxic convulsions and hypoglycemia due to tetramethyl succinonitrile (TMSN) exposure are reported. Material and Methods: Forty-four workers exposed to TMSN in the PVC production plant participated in occupational health medical check-ups including medical history, clinical examination and clinical chemistry. A 4-year follow-up was performed. To evaluate occupational exposure, measurements of TMSN in the ambient air as well as personal air sampling were conducted. Results: Four workers suffered from convulsions with reversible pathologic EEG and 16 other persons were hypoglycemic. Other frequent symptoms included headaches, dizziness and unpleasant taste sensations. TMSN levels had been clearly above the Swiss occupational exposure limit value (MAK). Occupational hygiene interventions resulted in a reduction of the TMSN concentration below the MAK value. TMSN related symptoms have not been observed anymore in the 4-year follow-up. Conclusion: TMSN is a convulsive substance which in humans has also a hypoglycemic effect.