[en] Iodine-131 was found to dominate the gamma spectra of dried sludge and concentrated liquid effluent samples from a sewage treatment plant serving the area containing the Texas Medical Center in 1975. The partition of ¹³¹I between the dried sludge and the liquid effluent was found to be reasonably constant during the sampling period. Knowledge of the concentration in these phases and the daily output of sludge and effluent led to the estimate of an annual discharge of over one curie of ¹³¹I. Circumstantial evidence linking this discharge to the practice of nuclear medicine in the area is considered strong, but an attempt to correlate variation in the effluent activity with actual nuclide use was inconclusive. An analysis of possible exposure pathways indicated that negligible human exposure resulted from the ¹³¹I released primarily because the receiving body of water is a saline industrial waterway