Thirty-four patients with progressive low grade non-Hodgkin's lymphoma were treated with 1 microgram oral alfacalcidol daily. Complete response was seen in four patients and a partial response in four patients with an overall response rate of 24%. Median duration of response was 14 months. Disease stabilised in ten other patients (29%) and 16 patients (47%) had tumour progression. In the sub-group of patients with follicular, small-cleaved cell-lymphoma the overall response to treatment was 29%. Apart from one patient who had a mild transitory elevation of serum calcium there was no recorded toxicity from alfacidol. These results indicate that alfacalcidol has significant antitumour activity in patients with low grade non-Hodgkin's lymphoma of the follicular, small-cleaved cell type.