Yttrium-90 synovectomy resulted in complete resolution of chronic knee synovitis in 40% of a group of 15 patients. These patients would have otherwise required surgical synovectomy. The number of chromosomal aberrations detected in 30 patients after yttrium-90 synovectomy showed a statistically significant increase over the pre-treatment analysis, and, following treatment, chromosomal damage was noted in nine patients who had no damage before treatment. The risk arising from this chromosomal damage is uncertain but, while the long-term implications of the chromosomal aberrations are investigated in those who have already been treated, this therapy should be reserved for the older patient.