A problem associated with some patients undergoing hemodialysis is the presence of muscle cramps. As the etiology of these cramps is still unknown, the purpose of this study was to evaluate tonic muscle activity and muscle cramps during hemodialysis. Forty-six complete dialysis treatment sessions were studied. Electromyographic (EMG) activity was recorded from a leg muscle in patients who cramped and control patients who did not cramp. EMG was analyzed via a minicomputer for each minute of recording time. Results indicated the mean muscle cramp latency from start of dialysis was 248 minutes. Average muscle cramp was 10 minutes in duration and took three minutes to develop and seven minutes to fully dissipate. Tonic EMG activity in patients with muscle cramps showed a continued increase throughout the latter part of dialysis whereas the control patients showed a continual decline. The results suggest that the latent increase in EMG activity may be causally related to muscle cramps and may be a useful predictor as to the onset of muscle cramps during hemodialysis.