The relationship between positron emission tomography (PET) findings and developmental or seizure outcome was examined in 17 infants (11 males, six females; mean age at onset of spasms 7 months, range 3 to 26 months) with newly diagnosed cryptogenic West syndrome. The predictive value of PET in these infants was assessed. PET was performed in the infants at the onset of spasms and 3 months after initial therapy using 18F-labelled 2-deoxy-2-fluoro-D-glucose. A third PET was performed at 18 months of age if the second scan was abnormal. All infants were followed up until at least 3 years of age. Cortical hypometabolism was detected in 11 infants on the first PET and in five infants on the second. Rate of developmental delay at the last follow-up was significantly higher in infants with hypometabolism on the second PET than in those without PET abnormalities (p<0.05). Rate of seizure occurrence after initial treatment was higher in infants with cortical hypometabolism on the second PET, but the difference was not statistically significant. Results suggest that when PET after the initial treatment shows no abnormalities, even though the first PET shows hypometabolism, infants with cryptogenic West syndrome may have a favourable developmental or seizure outcome. PET may be a useful tool in evaluating the prognosis in infants with cryptogenic West syndrome.