Three of the waterborne disease outbreaks in 1979 typified problems common to many water systems. Investigation of the outbreaks identified as causative factors deficiencies in water treatment practices and plant operation and a breakdown in surveillance activities designed to protect public health. Preliminary information indicates that a total of 45 outbreaks occurred during 1979, increasing the annual average of outbreaks to 36 for the four-year period from 1976 to 1979. This represents a 50 percent increase over the 1971-75 average of 24. The five-year averages have steadily increased from an annual average of ten during 1951-55. Before that period the trend was declining.