We have tried to estimate the toxic potential of waste from nuclear power plants and from power plants burning fossil fuels. The potential risks have been expressed as 'risk potentials' or 'person equivalents'. These are purely theoretical units and represent only an attempt to quantify the potential impact of different sources and substances on human health. Existing concentration limits for effects on human health are used. The philosophy behind establishing limits for several carcinogenic chemicals is based on a linear dose-effect curve. That is, no lower concentration of no effect exists and one has to accept a certain small risk by accepting the concentration limit. This is in line with the establishment of limits for radiation. Waste products from coal combustion have the highest potential risk among the fossil fuel alternatives. The highest risk is caused by metals, and the fly ash represents the effluent stream giving the largest contribution to the potential risk. The waste from nuclear power production has a lower potential risk than coal if today's limit values are used. If one adjusts the limits for radiation dose and the concentration limit values so that a similar risk is accepted by the limits, nuclear waste seems to have a much higher potential risk than waste from fossil fuel. The possibility that such risk estimates may be used as arguments for safe storage of the different types of waste is discussed. In order to obtain the actual risk from the potential risk, the dispersion of the waste in the environment and its uptake and effects in man have to be taken into account.