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The health risk posed by effluent irrigation in a semi-arid area of Portugal was assessed. The residual contamination found on lettuces spray-irrigated with the effluent from a conventional trickling filter plant was compared to that of locally marketed lettuces and to the ICMSF (1974) standards. The 2-year study indicated initially high levels of faecal indicator bacteria on lettuces, but after irrigation ceased, the restricting climatic conditions sufficed to decontaminate them to local market quality within 5 days when no salmonellae could be detected. Although high, E. coli I levels were within the tolerance range of the ICMSF standards suggesting negligible excess exposure of consumers to faecal contamination from the effluent-irrigated lettuces.