The data of 946 skin cancer patients treated with single doses or with 4, 8, 17, 40 or 47 fractions of X-rays were analysed using Strandquist's formalism and the linear-quadratic model. The analysis of tumour control in relation to tumour size and fractionation schedule shows a strong correlation between tumour size and tumour control rate. For small tumours, single dose irradiation was as effective as fractionated treatment. For large tumours, a straight line was easily fitted to the TCD50 and TCD90 values plotted logarithmically against time or number of fractions; the exponent for overall treatment time was 0.27 and for number of fractions it was 0.31. With the linear-quadratic model, an alpha/beta ratio of 13.8 Gy was calculated. On the assumption that the number of clonogenic cells in the tumour increases in proportion to tumour volume, Do values between 0.76 and 1.33 Gy were calculated and cellular survival curves were constructed. The estimated clonogenic fraction of tumour cells is about 10(-3); no influence of hypoxic clonogenic cells on local tumour control with single doses could be demonstrated.