In the Netherlands about 4 million people (283/1000) suffer from gastroenteritis every year, of which 500,000 cases are caused by 'Norwalk-like viruses' (NLVs), formerly known as 'small round-structured viruses'. The reports of two outbreaks illustrate the difficulties in determining the cause and source of the infection. The course is usually mild, but complications may be serious and ought to be documented. Vomiting and diarrhoea are the prominent signs and dehydration is the most common complication. Strict hygiene is warranted to prevent spreading of the disease. NLVs are highly infectious, notably via the faecal-oral route or by aerosols generated by vomiting. Fecally contaminated seafood and other food components that are not heated are an important source of infection, the main vehicle being sewage water. The microbiological quality control of food is often still based on bacteriological contamination, and therefore viral contamination may remain unnoticed. Reverse transcriptase PCR is a recent diagnostic tool.