de Meer et al. BMC Public Health 2010, 10:615 http://www.biomedcentral.com/1471-2458/10/615 and growth, and screening activities (for example conge- nital disease, vision impairment, hearing loss). Participa- tion rate to PCH routine examinations exceeds 90% without differences by gender, parental education, family income, and ethnic background [13]. Postal questionnaires were sent to a national sample of parent-child couples together with the invitation for a routine health examination by PCH. Parents completed questions on background characteristics, the child’s phy- sical health, and behaviour problems. The child question- naire comprised data on smoking, bullying, and friends. Parents and children completed the questionnaires at home and delivered them in a sealed envelope at the PCH clinic when coming for the child’s preventive health examination. The study protocol was approved by the local Medical Ethical Committee including verbal informed consent by parents to the child health nurse that took in the sealed questionnaires. Study population The study population was obtained by means of a two- step procedure. First, a random sample of child-health- care services was drawn, after stratification by region, urbanization ethnicity. Subsequently, for each PCH clinic a random sample of 100 children, grade 5-8 of primary school (age 7-13). After correction for stratified sampling, the samples were representative for the entire Dutch population in terms of income, type of family and paren- tal educational level. Out of the 1706 invited children 1350 (79%) parents gave informed consent for participation in the study. Most important reason for non-participation was a lack of interest in the study (37%). Children with missing data on sex (N = 46) or age (N = 4) were excluded. Since smoking did not occur among children under 9 years of age, we restricted the population to children aged 9 and older (N = 1230). Background characteristics Parents completed questionnaire data on the child’s and family’s background characteristics including sex, family composition, ethnic origin, parental education, and family income. Smoking Children were asked if they ever had smoked cigarettes, categorized as ‘never’, ‘once or twice’, ‘regular’, or ‘daily’. For the current analysis we dichotomized the answers in ‘never’ and ‘ever’, since only 4 children reported ‘regular’ or ‘daily’ smoking. Child problem behaviour Parents completed the 120 problem items of the validated Dutch version of the Child Behaviour CheckList Page 2 of 6 (CBCL) [14,15]. For the current analysis, the CBCL smoking item was deleted. Raw CBCL scores were com- puted for eight syndrome scales, i.e. attention problems, aggressive behaviour, anxious/depressed, delinquent behaviour, social problems, somatic complaints, thought problems, and being withdrawn. Broadband groups of syndrome scales comprised internalizing (anxious/ depressed, somatic complaints, withdrawn) and externa- lizing problems (aggressive or delinquent behaviour). All CBCL-scores were categorized in tertiles of the gender- specific distribution indicating three levels of problem behaviour. Statistical analysis Statistical analyses were performed using the SPSS 12.0 statistical software package. Relationships between smok- ing initiation and tertiles of CBCL-scores were tested using Chi-square tests. Crude and multiple logistic regres- sion analyses were performed. To explore differences by gender we performed stratified logistic regression analyses for sex. Results Of the 1230 children aged 9-13 years, the CBCL was completed for 1118 children and additional data on smok- ing were available for 960 children (78.0%). Ever smoking was reported by 57 (5.9%) children. Children that reported ever smoking were older (P = 0.022) and less frequently had a highly-educated mother (P = 0.026) compared to never smokers. There was no difference between ever and never smokers for sex, ethnic background, family compo- sition, family size, and father’s education (table 1). Table 2 shows the distribution of smokers by tertiles of CBCL score. Among girls, a positive trend was observed for a report of smoking and a CBCL score for total pro- blems (P = 0.062), delinquent behaviour (P = 0.050), attention problems (P = 0.010), and thought problems (P = 0.039). Except for thought problems, the higher pre- valence rates only occurred for the uppermost tertiles of CBCL-scores. For boys we did not observe any relation- ships of smoking with CBCL-scores. Logistic regression analyses adjusted for age, sex and mother’s education yielded statistically significant odds ratios for thought problems and delinquent behaviour (table 3). Gender stratified logistic regression were per- formed for associations with P < 0.100 in bivariate (table 2) or multivariate analyses (table 3), i.e. total problems, delinquent behaviour, attention problems and thought problems. In boys, none of the CBCL problem scores was associated with smoking initiation (data not shown). In girls, taking up smoking occurred more frequently in girls with a higher CBCL problem score for thought problems, attention problems, or delinquent behaviour (table 4).