The head sizes of 38 patients, growth hormone (GH) deficient following craniospinal (n = 26) or cranial irradiation (n = 12), have been assessed before (n = 38) and on completion of GH therapy (n = 15) or at the end of a similar period of observation without GH (n = 7). These results were compared to the change in head size seen in idiopathic GH deficiency following GH therapy (n = 14). Before GH therapy, the latter had small heads [mean occipitofrontal circumference SD score (SDS) -1], which were relatively large compared to the height deficit [height SDS (CA) -4.7], and they exhibited catch-up growth with GH (delta occipitofrontal circumference SDS + 0.7, final occipitofrontal circumference SDS -0.2). In contrast, over a similar period all patients, who previously had received cranial irradiation in the dosage range 2700-4750 centi-Geigy, irrespective of the radiation schedule or GH treatment, showed a decrease in occipitofrontal circumference SDS (mean delta -0.9), a significant difference to the expected head growth of normal children over a similar period (p less than 0.01). We have noted that restricted head growth occurs in the years following cranial irradiation and is unaffected by GH therapy. Earlier work has shown that cranial irradiation may impair intelligence. The exact relationship between intellectual impairment and stunted head growth remains to be determined.