The efficacy and suitability of photodynamic therapy (PDT) was compared with that of cryotherapy in the treatment of 40 lesions of Bowen's disease. Lesions were randomized to receive either cryotherapy with liquid nitrogen, or PDT using a portable desktop lamp incorporating a 300 W xenon short arc discharge source. A porphyrin precursor, 5-aminolaevulinic acid (5-ALA), was applied topically 4 h before irradiation in the PDT group. Each lesion received 125 J/cm2 at a fluence rate of 70 mW/cm2. All patients were reviewed at 2-monthly intervals and treatments repeated if required. Cryotherapy produced clearance in 10 of 20 lesions after one treatment, the remaining 10 lesions requiring two or three treatment applications. PDT resulted in clearance of 15 of 20 lesions after one treatment and of the remaining five lesions after a second treatment. The probability that a lesion cleared after one treatment was greater with PDT than cryotherapy (P < 0.01). Cryotherapy was associated with ulceration (five of 20), infection (two of 20) and recurrent disease (two of 20); no such complications occurred following PDT. PDT using a non-laser light source and topical 5-ALA appears to be at least as effective as cryotherapy in the treatment of Bowen's disease with fewer adverse effects.