Cases of cryptosporidiosis in patients with the acquired immunodeficiency syndrome (AIDS) residing in Melbourne over a 6-year period (1990-1995) are described. During this period 85 cases occurred, while 979 new AIDS diagnoses were notified. Over this period temporal clustering in cryptosporidial detection was evident (P=0.007), but the pattern was not statistically associated with the season, rainfall (P=0.88), mean average maximal temperature (P=0.15) or mean average minimal temperature. Further studies should identify these risk factors and provide an opportunity to prevent this devastating disease.