BACKGROUND Long-term use of topical trimethylpsoralen (TMP) psoralen bath plus ultraviolet A (bath PUVA) is considered safe with regard to the risk of skin cancer. However, the potential for severe phototoxicity limits its use. OBJECTIVE To study the effect of dilution of the TMP bath on the minimal phototoxic dose (MPD). METHODS Fifteen volunteers participated in the study. The MPD tests were performed for three TMP concentrations: 0.33 mg L-1, 0.1 mg L-1 and 0.033 mg L-1 at 2-week intervals. Geometric UVA dose series increasing by a factor of radical2 were used for the testing on the previously unexposed buttock skin. The MPD72 h was assessed at 72 h from the bath. RESULTS For the highest TMP concentration of 0.33 mg L-1, the median MPD72 h was 0.14 J cm-2 (95% confidence interval (CI), 0.10-0.14 J cm-2). For the diluted TMP bath concentration of 0.1 mg L-1, the median MPD72 h increased to 0.29 J cm-2 (95% CI, 0.2-0.41 J cm-2) and for 0.033 mg L-1 to 0.81 J cm-2 (95% CI, 0.57-1.15 J cm-2), respectively. Thus, diluting the labelled concentration of 0.33 mg L-1 1 : 10 increased the median MPD72 h 5.6-fold. CONCLUSIONS With regard to the safety and practicality of the TMP bath PUVA, the lower concentrations of TMP may be of clinical importance, and this needs to be validated in future controlled clinical trials.