Developmental deviance is known to be associated with schizophrenia. We tested the hypothesis that the most severe schizophrenia requiring treatment with clozapine would particularly show these effects. Therefore, associative factors from pregnancy, delivery, neonatal and socio-demographic characteristics were compared between the clozapine-treated schizophrenic cases (n = 17) and the remaining cases of schizophrenia treated with typical neuroleptics (n = 59) from an unselected, general population North Finland 1966 Birth Cohort, (n = 11,017). By the end of 1994, seventeen (22%) of a total 76 DSM-III-R schizophrenia patients, had received clozapine treatment. The mean length at birth was 52 cm in the clozapine group (in the non-clozapine group 50 cm) and correspondingly the mean one year weight 10.4 kg (9.8 kg), with the differences between the groups being statistically significant (P < 0.05). Other pregnancy, delivery, and socio-demographic characteristics were, however, similar in the clozapine-treated and the remaining cases with schizophrenia. There was no evidence that severe schizophrenia requiring treatment with clozapine was associated with impaired intrauterine or post-natal development. Indeed, during the first years of life the former group was larger in terms of weight and length than those treated with typical neuroleptics. These early developmental characteristics did not predict clinical severity of schizophrenia in adulthood.