Possible undetected outbreaks of cryptosporidiosis in areas of the north west of England supplied by an unfiltered surface water source.

We report a ten-year retrospective analysis of laboratory reports of cryptosporidium infection in the North West of England. Weekly report data from six health authorities known to have been affected by outbreaks associated with a single supply were compared with data from other health authorities in the North West. Following graphical representation of report rates, it would appear that outbreaks in the six health authorities were considerably more common than the average recorded in the national outbreak surveillance system.