Analysis of chromosome breakage with mitomycin C (MMC) and folate-deficient culture conditions was undertaken on 18 Prader-Labhart-Willi syndrome (PLWS) patients (10 with 15q12 deletion [5 females, 5 males; mean age = 17.9 yr, range of 0.3 to 40 yr] and 8 without deletion [2 females, 6 males; mean age = 18.6 yr, range of 7 to 26 yr]), 21 PLWS parents with an average age of 39.2 yr and a range of 25 to 70 yr (12 fathers [8 fathers of PLWS children with the 15q12 deletion and 4 fathers of PLWS children with normal chromosomes] and 9 mothers [4 mothers of PLWS children with the 15q12 deletion and 5 mothers of PLWS children with normal chromosomes]), and age-matched control individuals. There was no difference between PLWS patients and control individuals in the number of chromosome and chromatid aberrations in cells grown at 48 and/or 96 hr in either 20 ng/ml or 50 ng/ml of MMC or between the PLWS parents and control individuals in cells grown in 50 ng/ml MMC for 96 hr, although a small increase (P less than 0.05) in chromosome breakage was found in cells from the total PLWS parental group compared with control individuals exposed for 48 hr in 50 ng/ml MMC. There was also no significant difference in chromosome fragile site frequency in cells grown in folate-deficient culture conditions in PLWS patients, their parents, or controls.(ABSTRACT TRUNCATED AT 250 WORDS)