BACKGROUND: The importance of a person's perceptions about the causes of their disease has been emphasised by research on various diseases. Several studies have found perception may be linked to protective behaviours. OBJECTIVE: This study intends to identify the main perceived causes of sporadic cryptosporidiosis, and to analyse some of the factors that may influence respondent's perception. The role of respondents' attributions, the scientific plausibility of perceptions, and the importance of specific information sources are also explored. DESIGN: Quantitative and qualitative analyses of data from a case-control study. SETTING: General population in Wales and north west England. PARTICIPANTS: The study is based on a sample of 411 respondents from Wales and north west of England, whose cryptosporidiosis diagnosis was confirmed by a laboratory. RESULTS: The results show that the most frequent perceived causes are water (by drinking it or swimming), contagion (mostly from children), and contaminated food. Perceived causes are qualitatively similar to the ones described in scientific literature, but some quantitative differences are evident. Respondents' certainty in relation to the cause of illness is directly related with plausibility. The most frequent information sources used by respondents were test stool results, environmental health officers, and doctors or nurses. Results suggest that information sources may influence the perception of the causes of cryptosporidiosis. Qualitative data provided a few clues about situations where sporadic and outbreak cases may be confused. CONCLUSION: In contrast with outbreaks, various information sources in addition to the media are used by people with sporadic cryptosporidiosis that in turn affects the perception of aetiology. This has implications for the dissemination of information about control measures for cryptosporidiosis and surveillance activities.